1. Introduction

Against the background of globalization, the importance of second language acquisition is increasingly prominent, and the effect of vocabulary acquisition has always been the core issue of concern in the academic circle. Although the traditional learning mode can accumulate vocabulary, it takes a long time. In contrast, incidental vocabulary learning, which can achieve a large amount of vocabulary accumulation in a natural context, has gradually become a research hotspot in the field of second language teaching.

Li and Wu argue that variant contexts outperform repetitive contexts in terms of word meaning comprehension and long-term memory, but this effect is influenced by the type of task and the proficiency level [1]. Furthermore, although the popularity of multimedia technology has expanded the forms of context (such as video, audio), unreasonable use of multimedia may distract attention and weaken the learning effect [2]. How to balance the richness of context and learners' cognition has become an urgent problem to be solved. Therefore, from the perspective of context, the mechanisms by which variant contexts and repetitive contexts affect vocabulary acquisition still require in-depth research.

In terms of different task types, reading task, listening task, viewing task and communication tasks present different learning ways. Reading tasks rely on the textual context and emphasize the construction of vocabulary networks through word.

guessing strategies and the high-frequency repetition of words [3]. For the lower-level students, simple texts are more suitable and overly complex texts are not suitable for reading. For the higher-level students, more challenging texts are more suitable to improve their own level. The listening task strengthens memory by the high frequency of the appearance of the key words [4]. For the general learners, there is a greater preference for listening tasks without visual interference. The viewing task reduces the difficulty of understanding through the combination of audio-visual elements and is particularly suitable for learning concrete vocabulary [5]. Learners can choose movies without subtitles or with bilingual subtitles and so on, which can achieve a better combination of audio-visual experience. Communication tasks involve interactive output of vocabulary usage to enhance memory retention rates [6].

Communicating with native speakers or those proficient in a second language can lead to a good learning approach that combines input and output for the average learner. The key point is that the impact of different types of tasks on students of different learning levels still needs to be further studied.

This introduction aims to systematically sort out the contents of existing studies on the above-mentioned variant contexts and the acquisition of incidental vocabulary by different task types, point out the limitations of the existing theories, and propose the directions that future research should focus on, with the expectation of providing more comprehensive theoretical support for the formulation of differentiated strategies in second language teaching.

2. Literature review

2.1. The influence of variable context on incidental vocabulary learning

In reading, context is an important factor affecting the acquisition of secondary vocabulary by second-language speakers. Li Zan and Wu Shiyu [1] conducted experiments on the impact of variable context and repeated context on vocabulary learning. In the experiment, the ability of non-English majors to acquire vocabulary in reading is sometimes not affected by a single factor, but by multiple factors intertwined, This is mainly for due to subjects' comprehension of the story and context. In semantic generation tasks, the role of context is more obvious, and it can be refined into mutated context and repeated context. In the test of this task, it can be seen that the scope of influence of story understanding ability is relatively small, but the effect of vocabulary learning under repeated context, such as word understanding and memory duration, is far from that of the variable context, and regardless of the test timing, results remain consistent. In the complete fill-in-the-blank task, the three factors of testing time, context and subject's ability to understand the story show the complex effect of interaction. Although the test results still show that the learning effect in the changing context is stronger than that in the repeated context, this is limited to the stage where the subject's ability to understand the story is low. On the contrary, the gap will be narrowed. Therefore, it can be seen that in the complete fill-in-the-blank test, which places greater cognitive demands on bilinguals, if you want to get better learning results, you can't rely solely on changes in the context. Therefore, for different levels of learning goals, such as: learning of explicit knowledge from the context, learning of vocabulary knowledge proficiency under clues, the role of variable context and understanding ability is different. But if want to pursue long-term memory and learning, you should combine the two.

In recent years, the use of multimedia has promoted the change of context in second language teaching. Compared with a single incidental learning of vocabulary through reading annotated texts, the experiments of AI-Seghayer and Akbulut both show that the multimedia text is more effective. For example, the intervention of video. The effect of context changes brought about by multimedia on vocabulary acquisition is not only reflected in reading, but also in listening. Liu [7] In her study on the impact of multimedia on the vocabulary acquired in listening, 90 students with intermediate level majoring in English from an undergraduate institution were selected to test using the Nation vocabulary test form and special four listening words and concluded that although multimedia annotations do not promote listening comprehension Such as text annotations, but it can strengthen the subject's memory of vocabulary before and after listening. In addition to traditional methods, other cognitive methods can also play a role in promoting the change of context, such as watching foreign language movies. Watching movies for a certain period of time and learning the meaning of words based on the context avoids the boring repetition of traditional memorizing words.

It is worth noting that although the widespread use of multimedia in second-language teaching enhances the diversity of learning contexts, the changes brought about are not necessarily positive. For example, the unreasonable use of multimedia in the listening process may distract the attention of second speakers, which can impair their comprehension abilities. For example, if the second language person's own language ability is weak and the vocabulary accumulation is small, the goal of acquiring complex vocabulary cannot be achieved by watching movies alone [2] . Therefore, the use of multimedia should depend on the situation.

Affected by the language environment, teaching mode, language habits and other factors, most second-linguals are far inferior to native speakers in terms of reading volume, and the lack of vocabulary hinders the reading ability of second speakers. When learning unfamiliar words for the first time, second speakers are prone to forgetting, so timely consolidation and review become essential. Learners can combine the theory mentioned in the "typical form of forgetting", and repeat the words for the first time as soon as possible, and the subsequent repetition can be subsequent repetitions can be spaced increasingly over time, so that it enters the long-term memory system. Since repetition is an inevitable requirement, is it easier for second speakers to repeat unfamiliar words in the same article or to make strange words constantly appear in multiple different articles? Existing studies have found that on the basis of a certain vocabulary foundation and language ability, using diverse reading context and rather than a single vocabulary source can deepen the understanding of vocabulary by increasing the breadth and quantity of reading [8]. At the same time, because the secondary words learned by bilinguals in reading are often guessed by referring to the context, the variable context undoubtedly provides bilinguals with multiple opportunities to speculate and remember. Secondly, reasonable annotation methods can also guide bilinguals to enhance their understanding and memory of vocabulary in combination with the context, which is suitable for students who find it difficult to get the meaning of words in reading in a non-native context. However, no matter which form, the low frequency of unfamiliar words or repeated exposure in a single context cannot achieve the best effect. It should be combined with the variable context to enhance the richness to improve the efficiency of learning. This conclusion is similar to that of Li Zan and Wu Shiyu [1], who also emphasize the importance of changing different contexts. It is worth noting that the selected reading text should be focused on. Based on the principle of personalized needs, bilinguals can choose the text type they are interested in. The necessity of combining personalized needs with variable context is also reflected in bilingual teaching. In bilingual teaching, vocabulary learning is an important issue, especially professional vocabulary. Not only the vocabulary itself is more difficult, but also the vocabulary of students is relatively small. Therefore, vocabulary acquisition plays an important role in promoting bilingual teaching. Teachers can reasonably master the usage of vocabulary by guiding students to complete relevant tasks in authentic or semi-authentic contexts. According to the principle of mass contact, teachers should try to teach under certain context conditions, not only to pay attention to interests, but also to rely on the variable context to provide schools with more opportunities to predict the meaning of words [9].

2.2. Different types of tasks of incidental vocabulary learning

Different types of tasks have different influences of incidental vocabulary learning. The most common tasks are reading task, listening task, viewing task, and communication task.

2.2.1. Reading task

The reading mentioned here refers to books, documents, and other types of obtaining content through text types. Teng said reading give an opportunity to interact with, exposure to rich contexts and vocabulary, it can lead to process of incidental vocabulary learning [10]. Although reading has a great impact on incidental vocabulary learning, but it is very important to choose the context, it is particularly necessary to find a reading text that suits one's own level and interests. According to the applied linguistics, it has been reported that if a learner knows 95% to 98% of the words (tokens) in a text, then they can learn new vocabulary through incidental learning by guessing the meanings of unfamiliar words based on their context [3].When learners select high-quality texts that truly meet their own needs from numerous texts for reading carefully and wisely based on their own knowledge reserves, reading abilities, and learning goals, during the process of concentrating on reading the texts, they will not rush to turn to dictionaries as soon as they encounter unfamiliar words. In contrast, they will fully mobilize their existing language knowledge and context understanding ability, skillfully apply the meanings of roots and affixes, combine the logical relationships in the context, and refer to synonyms and antonyms that appear in the text to boldly guess the meanings of words. Then, in this process full of exploration and challenges, they can efficiently learn new vocabulary and gradually enrich their vocabulary. After initially mastering new vocabulary through the word-guessing strategy, continuous and extensive reading will form a virtuous language input cycle. With the steady accumulation of reading volume, learners will find that the target vocabulary repeatedly appears in different parts of speech, collocations, and contextual forms. Each repeated encounter is a cognitive reinforcement. Especially when these words are repeatedly distributed in texts on different topics, vocabulary is the knowledge point that connects cultural background, grammatical rules, and pragmatic habits. Ultimately, learners can not only accurately extract the meanings of words, but also naturally invoke them when expressing, transforming passive vocabulary into genuine language application ability.

2.2.2. Listening task

Compared with the reading task, there are few studies on incidental vocabulary learning in listening tasks. Actually, listening is one of the most ways in learners’ daily life. For instance, English songs that learners can hear at any time while walking on the street, English blogs on the radio, and even native speakers and teachers with standard pronunciation that they encounter in daily life. The input method of the Listening task can enhance the repetition exposure rate of words. For example, simple English songs will repeat the key words multiple times within a limited few minutes, and the repetition exposure rate of words will also increase during the process of repeatedly listening to the songs. Most studies show that the more frequently learners are exposed to key words, the more likely they are to be learned [4]. Whether it's songs, blogs, lectures, etc., this approach has a common feature - it revolves around one theme. When the content is limited, vocabulary is concentrated in the conjunctions of key words, and thus the efficiency of learning words will also increase. In addition, the Listening task can promote learners to focus more on the recognition of vocabulary, grammar and semantics. When learners participate in listening tasks, their attention will be highly concentrated on the language information they hear. In this state of concentration, learners will analyze vocabulary actively and strive to identify those familiar or strange words. At the same time, they will subconsciously conduct grammatical analysis on the sentences they hear, learn and apply idiomatic grammatical forms. Furthermore, learners will have an in-depth understanding of the semantics and grasp the meanings conveyed by the sentences. It is worth noting that in the absence of visual assistance, the effect of this kind of focused learning will be more significant. In the absence of interference from visual information, learners' auditory perception will become more acute. Erber argues that by changing the visual optical articulation, the study explores its influence on speech audiovisual perception, which mainly depends on hearing under extremely poor optical conditions [5]. For example, learners cannot guess the meaning of words by observing their spelling. They can only identify words by relying on their familiarity with pronunciation and language sense, which makes their memory of words more solid.

2.2.3. Viewing task

The Viewing task and the reading task are different. Unlike text reading in the reading task, the viewing task mainly relies on vision and hearing to receive information. For example, when watching English movies, TV series and so on, learners are simultaneously exposed to pictures, sounds and possible subtitles. This kind of input method that combines audio-visual elements makes it easier for learners to carry out in their daily lives. Fakhr et al. argues that watching full English TV programs with subtitles is a great aid to incidental vocabulary learning [11]. The study selected 84 students, let the experimental group watch the complete English TV program with subtitles, the control group did not watch, and then conducted a vocabulary test on them, the results showed that the experimental group was significantly higher than the control group in learning and remembering words. Through pictures and context, learners can quickly understand the general meaning of words. The learning effect is better for words with obvious visual images, such as "apple". When learners see the picture of a bright red apple on the screen and hear the corresponding pronunciation at the same time, they can easily connect the word with the real object, and the learning effect is significantly improved. Meanwhile, this approach is also more suitable for second language learners at any stage. As long as learners have carried out the viewing task and are in the context of audio-visual combination, it is highly likely that they will learn acquire unfamiliar words and continuously expand their vocabulary. Green argues that watching many children's entertainment programs with subtitles is rich in early reading vocabulary, and has potential for the development of phonological, semantic, and print knowledge of early reading vocabulary [12]. Therefore, choosing to watch English cartoons with subtitles for children's English enlightenment is a very effective way, and its effect is often better than that of ordinary English learning methods. It can not only stimulate children's interest in learning English, allowing them to come into contact with English in a relaxed and pleasant atmosphere, but also improve the efficiency of vocabulary learning through the synergistic effect of multiple senses, laying a solid foundation for children's future English learning.

2.2.4. Communication task

Compared with the first three tasks, the communication task introduces an interactive learning method involving communication with others, that is, an output method. Newton argues that communicating information orally without resorting to shared written materials necessarily requires paying more attention to accurate perception [6]. Communication provides people with a large number of opportunities to come into contact with new words in context. When people engage in conversations, read texts, or listen to others speak, they come into contact with all kinds of words that they might not have encountered originally. This kind of contact is crucial for incidental vocabulary learning because it enables people to see and hear the practical application of new words, which helps them understand the meaning of the words and their usage in different contexts. This kind of real-time interaction not only tests language comprehension ability, but also forces learners to quickly retrieve and apply vocabulary in dynamic conversations, thereby forming high-intensity cognitive stimulation. Meanwhile, communication tasks can be the motivation for learning English. When people participate in communication, they usually have the motivation to understand others and make themselves understood. This kind of motivation will promote them to pay more attention to new words and strive to learn them. For example, in a classroom similar to bilingual teaching, when students communicate with the professor, they will do their best to express themselves. Students will express the content they want to express as perfectly as possible with words and grammar. During this process, incidental vocabulary learning will be carried out. Furthermore, through communication, learners can interact with native speakers or those with a higher language proficiency, which helps them improve their language skills, such as oral communication ability, idiomatic and accurate writing ability, and precise reading ability. Social interaction also provides learners with opportunities to ask questions, receive feedback and learn from others.

3. Discussion

In terms of the application of the theory of vocabulary acquisition in English teaching, many scholars have carried out practice and discussion. Simamora and Oktaviani focus on the combination of English teaching and variable context. In the classrooms they designed, students learn English vocabulary by viewing preferred movies. The results show that students can effectively increase their vocabulary by watching movies, and can better understand the meaning of words through the expressions and movements of the actors. For example, Sari and Aminatun deeply analyzed the use of media in the process of English teaching to attract students to learn languages. The two have a positive attitude towards the method that students watch movies in class to enhance their vocabulary. Research shows that watching movies can facilitate vocabulary improvement and ultimately use it in conversation or writing when using English. This means that the variable context not only allows second speakers to acquire the surface meaning of words, but also deepens their understanding of the specific use of words.

In addition to watching movies, songs can also assist vocabulary acquisition to some extent. Gushendra focuses on using songs to improve students' vocabulary. The study found that students who learn English by listening to English songs demonstrate improved vocabulary retention. The influence of the mutated context is also reflected in the use of social media by second speakers. The above are some specific practices about vocabulary acquisition in teaching.

Starting from the influence of four relevant factors on the ability of secondary vocabulary acquisition, this article, combined with the theory of second language acquisition, systematically analyzes the time of vocabulary acquisition under different conditions, and stimulates the interest of secondary language teaching middle school students in vocabulary acquisition. Nowadays, the phenomenon of second language learners naturally acquiring vocabulary through context when reading has attracted more and more attention. Reading can not only improve speech comprehension, but also help learners unconsciously absorb new words. This accidental acquisition method has become the main method of vocabulary learning. The quality of the context has a significant impact on learning outcomes: different situations, repeated exposure of new words, and abundant context clues all help learners better grasp the meaning of vocabulary. Similarly, the effect of variable context may vary depending on the input methods, and listening input may be more adaptable to multilateral contexts. Therefore, studying the interaction of these factors can provide more comprehensive guidance for optimizing foreign language vocabulary teaching strategies.

In addition, in the four main aspects of the impact of different types of tasks on vocabulary acquisition, each aspect plays a different role. The role of Reading task is closely related to the prompts of context and the selection of text. Choosing articles suitable for bilinguals and fully mobilizing the ability of bilinguals to guess the meaning of words in combination with context is the key to Reading task. Listening task is frequently employed. It is the easiest to achieve among the four types of tasks, and often attracts bilinguals to concentrate through a fixed topic, making the learning process more focused. Compared with the first two, Viewing task is more like a combination, not simply listening or watching. This learning method of audio-visual combination is more suitable for words with obvious visual images. Unlike the first three, communication task adds output. This method is more conducive to stimulating the learning interest of second speakers.

4. Research limitation and prospects

Although this study clarifies the core role of context and task type in vocabulary acquisition, it still needs to be studied for the specific application of teaching strategies. Different development directions will help to establish a more scientific second language teaching model, naturally linking vocabulary learning with language application events.

The research on the interaction of factors is insufficient. Most of the current research focuses on the impact of a single factor (such as repeated exposure or a single task type) on the incidental acquisition of vocabulary and lacks in-depth exploration of the interaction of these factors. Although it is known that multiple factors may affect learners' vocabulary acquisition at the same time, the existing literature has not fully explored how these factors work together, thus affecting learners' vocabulary memory and application.

There is a lack of research on long-term effects. Most existing studies focus on short-term vocabulary acquisition and lack research on long-term effects. For example, whether learners can maintain the memory of these words for a long time and apply them effectively after repeated contact with words in a specific context has not been fully experimented. There is no discussion on long-term vocabulary retention and transformation, which limits the understanding of the continuity and depth of vocabulary acquisition.

5. Conclusion

This study reviews the key factors affecting the acquisition of collateral vocabulary by second-language learners, such as repeated exposure, mutated context, rich context and input methods (different task types). Although each factor has a significant impact on vocabulary acquisition alone, studies have shown that these factors are often intertwined and their effects are affected by the differences in learning this person. However, the in-depth discussion of the interaction of these factors in the existing literature is still insufficient, especially in the study of practical application scenarios and long-term effects. Future research should focus on how these factors work together in the natural language learning environment, and explore the impact of different task types and context changes on long-term vocabulary retention.

Authors contribution

All the authors contributed equally and their names were listed in alphabetical order.