1. Introduction

With the new curriculum reform, the education department has put forward new requirements for English learning: improving core English literacy, aiming at developing students' cross-language and cross-cultural communication skills and language skills [1], the learners' English communication and learning ability have also received extensive attention. According to Paul, in the daily communication of ordinary people, listening accounts for 45%, which is the key to daily communication, so listening teaching is an indispensable part of English learning. At present, there are many researches on listening teaching abroad. For example, according to Sasan Baleghizadeh’s investigation and study of the relationship among metacognitive strategy use, they found that using metacognitive listening strategies can effectively improve the listening performance, and suggested that teachers should consider the use of metacognitive during teaching to improve students' intrinsic motivation and performance [2]. Regarding the selection of listening materials, Wagner argues some scripted texts were written, revised, and then recorded with clear pronunciation and slow speech as material for listening study, which is not conducive to improving students' communicative competence. He believes that teachers should incorporate unscripted texts into the teaching of L2 listening [3]. Later, with the upgrading and popularization of electronic equipment, Aji suggested the blended learning method can effectively improve teachers' teaching efficiency and students' listening learning speed through the combination of online and offline [4]. We can see that many scholars have carried out research from different angles, which shows that this topic is very important.

However, the current research on listening teaching methods by Chinese scholars is in the process of exploration and development, and some theories and experiences are immature. Many teachers are still using traditional teaching methods, which leads to many problems in the current listening teaching, and the teaching effect is also significantly reduced. This can be seen from the following aspects. First, teachers are underprepared to effectively and explicitly teach listening skills. Listening is frequently tested, but rarely taught [5]. For example, scanning the questions to predict the content of material before listening, or making brief notes during listening process, and analyzing the reasons in detail after the listening, all of these can affect the improvement of students' listening level [6]. Besides, students do not understand the rules of pronunciation, so they can only accept the standard pronunciation in their mind, although they can understand the content of the listening materials, but they are very unfamiliar with the pronunciation in the tape, which makes it impossible to achieve intelligible input. Second, limited Listening Material. Most teachers choose listening materials from textbooks and tests, ignoring the integration and utilization of extracurricular teaching resources. Some tasks in the textbook are outdated and not very helpful to students, and the content are not necessarily suitable for all students of different majors and levels [7]. In addition, practice listening through exam questions frequently, which bring certain pressure to students' listening learning, and often cannot realize the effective play of English listening training. Third, Large Individual Differences. Different levels of foreign language learners may encounter different types of problems when learning listening [8]. This means that teachers need to have different teaching designs for different students. However, due to the traditional large-class teaching in China, the number of students is large, the level is uneven, and the class time is fixed. Among these objective factors, it is difficult to find a balance so that each student can get their own understandable input.

Based on the above discussion, it can be seen that China's English listening teaching lacks theoretical guidance. This paper briefly introduces the input theory and output theory, and proposes better teaching methods to improve students' listening ability based on the current situation of listening teaching in China.

2. Theory Background

2.1. Krashen's Input Hypothesis

In the late 1970s, the famous linguist Krashen put forward the second language acquisition monitoring model [9], and then in the mid-1980s, he further researched and concluded the “the input hypothesis model” [10], which includes five hypotheses, namely the acquisition and learning hypothesis, the natural order hypothesis, the input hypothesis, the monitor hypothesis and the affective filtering hypothesis. This article will focus on the input hypothesis. Krashen believes that a large amount of comprehensible input is conducive to improving the language ability of foreign language learners. The ideal language input should include the following characteristics.

Comprehensible. Krashen thought that it is necessary to input the comprehensible information to the foreign language learners during the second language acquisition [11]. If the learners can’t understand the input information, the languages can’t be acquired. Therefore, Krashen proposed the “i+1” structure. The “i” means the current language level of the language learner and “1” means the new language knowledge, which represents the difficulty slightly higher than the current level [12]. According to this trait, it is advantageous to have a foreign language environment to learn the new language for the beginners.

Interesting and relevant. Krashen thought that the input materials should be interesting and relevant to the daily life. If the input language information is interesting and closely related to the learner, it will greatly stimulate the learner’s interest in learning [11]. Inner emotion is an important factor affecting language acquisition, positive emotional attitudes make language learners more effective at learning language. Therefore, no matter the teaching materials or teaching content, interesting and relevant should be increasingly concerned.

Not grammatically sequenced. Krashen suggests that language acquisition is a natural process, while learning is unnatural. Acquisition focuses on the meaning of discourse materials rather than fixed forms, while learning pays attention to the rules of language, that is, grammatical structure. Therefore, in order to obtain ideal language input, input materials should not be rigidly bound to fixed forms.

Sufficient input. Krashen believes that language teachers should provide learners with sufficient language input materials in the process of acquiring a foreign language. Too little language input can lead to stagnant language skills of learners, and too much language input will make learners unacceptable and increase their psychological burden. Therefore, when students acquire a language, teachers should grasp the quantity of "1" and let foreign language learners gradually turn "1" into their own "i".

Teachers can teach listening based on Krashen's input hypothesis to ensure the effectiveness and comprehension of language input, and create a better learning environment for students.

2.2. Swain Output Hypothesis

Swain put “output hypothesis” in the early and mid-1980s [13]. After researching French immersion programs in Canada for 7 years, she found that in addition to the necessary comprehensible input, learners need to have a chance to export language in order to approach native speaker level. Before that, comprehensive input was always considered as the most important factor in second language acquisition. Swain argues that the output assumption has four major functions for language acquisition---noticing function, hypothesis testing function, metalinguistic function and fluency function [14].

Noticing function. Li Tao thought output draws learners' attention to language issues. Before language output, the learner should have enough cognitive resources to transfer or express the meaning of the sentences [14]. When the learner wants to output, he/ she will notice whether the form and meaning of his/ her language is reasonable and correct.

Hypothesis testing function. Hypothesis testing is predicated on interaction and feedback, no matter between learner and native speaker, learner and teacher or learners. When communicating with others, in order to explain or express clearly, the learner will improve his/ her original words to create new interlanguage [14].

Metalinguistic function. Swain considered that metalinguistic refers to the total knowledge of the learner in terms of language [11], i.e., the prototype of their knowledge about the structure, form, and other aspects of the language system that they acquire through reflecting on and analyzing language. The learner will pay attention to his/ her own language system and the analysis, then correct and output.

Fluency function. The more the learner output, more fluently the learner would speak. Learners reinforce existing knowledge and develop language processing automation in language use, learners reinforce existing knowledge.

3. The Application of Input-Output Theory in English Listening Teaching

3.1. Comprehensible Input

Without understanding input appropriately, learning simply cannot get any improvement [15]. Incomprehensible language is not only useless for learners, but may also have a negative impact. If a student does not fully understand the teaching material, he will not get effective input and fall into a learning dilemma [16]. Difficult listening will also increase anxiety. This emotion will not only hinder students' listening comprehension, but also make students afraid of learning listening, and even cause them to avoid learning. In order to improve the effectiveness of teaching, teachers need to make some changes to the listening input materials to meet the requirements of comprehensible input.

There are several aspects for modifying listening materials. First of all, the length of the material should be moderate. Too long material will not only burden students' memory, but also put pressure on students. Too short material cannot achieve the purpose of improving students' listening ability. Secondly, teachers should ensure that the words and sentence patterns in the listening materials are what the students have already learned, which is the basis for the students to understand the content. Third, teachers can slow down the speech rate of the recording according to the level of the students, such as slowing down the speech rate and changing the intonation through software settings, so that the listening content can be more clearly heard by the learners, which will also increase the students' sense of achievement in learning listening.

For individual students, teachers can select an "i+1" material based on their level. For large classes, teachers cannot identify a listening material that is suitable for all students. However, teachers can determine "i" according to the average level of most students in the class, this "i" can be produced through the combination of teachers' efforts and experience. For example, before listening, let students know the words, sentence patterns and material background in advance, so that all students can achieve the same level of known content. Such a unity is a certain mechanical unity. Moreover, due to the limitation of time and energy, we can only adjust a "1" that can be absorbed by most students in a limited framework.

Comprehensible input materials are not limited to classrooms. With today's advanced technology, we can access authentic listening input when we turn on our computer and our mobile phone. However, some English TV programs are more difficult for students, so teachers need to build a bridge between students and social English media, choose some suitable materials for students, so that students can improve their listening ability through their own efforts, to improve students' self-confidence. In this way, students can find a learning strategy that really suits them, and study with purpose, thereby improving their comprehensive English ability.

3.2. Interesting and Relevant Input

Krashen suggests that the more interesting and relevant the input material is, the more learners will be able to acquire language without realizing it. Only when the listening material is relevant to current events, something that students really enjoy and are interested in, will they listen more actively and attentively. In contrast, in our listening courses today, most of the listening materials are old and not up-to-date, and most of them are even ten years old. This material is boring and uninteresting to students. In fact, with the Internet, we now have access to interesting and up-to-date listening materials [17]. Teachers can choose from a wide range of listening materials on the basis of merit. Natural and authentic language input helps students to acquire language. In addition, a variety of input methods can also help to increase students' motivation, and teachers can make use of visual aids, such as showing VOA lecture videos in class to train students in listening.

3.3. Combining Acquisition and Learning

The Krashen 'input hypothesis' suggests that people have two separate ways of learning a language: acquisition and learning. Acquisition is learning subconsciously, which is unobtrusive, informal and natural, while learning is conscious language learning, which is obvious, formal, systematic and unnatural. To improve the effectiveness of listening teaching at this point, a combination of acquisition and learning is required.

The creation of an English language environment and the improvement of English language teaching methods are crucial in this regard. First and foremost, create a good English learning atmosphere, which let students acquire English more naturally in an English-speaking environment. To achieve this goal, teachers can use English as much as possible and carry out a large number of English communication seminars. Secondly, constantly innovate teaching methods. Teachers can stimulate students' enthusiasm for learning through scientific and technological means such as multimedia and computers [18].

3.4. Sufficient Input

Moving up from stage i to stage i+1 requires a sufficient amount of input as a prerequisite. Language learning is a process of quantitative to qualitative change. Only through exposure to a large amount of language materials can a strong sense of language be developed and a better grasp of the subtleties of the language be achieved, leading to a certain level of language competence.

Nowadays, the new teaching mode of English at university based on the Internet and computers is being promoted, providing favorable conditions for meeting the requirements of the amount of input for language learning, and teachers should take the initiative to adapt to this teaching mode [19]. In classroom training, teachers should increase the amount of class time available for listening courses so that students can learn to listen systematically under the guidance of the teacher, thus ensuring the quality and quantity of listening materials. In addition to classroom training, teachers should make full use of the Internet as a platform to provide a major number of listening resources for students' independent learning. Teachers can make full use of students' time outside of class so that they can use the Internet to guide students in acquiring a large number of language materials suitable for their listening abilities. The teacher can also provide individualized guidance to provide different quality language input for different language levels. At the same time, teachers should monitor and guide the amount of input to ensure that it is adequate. In addition, the development of listening requires a certain level of vocabulary and grammatical knowledge, so reading input is also important for listening, and teachers should try to increase the amount of reading students do.

3.5. Oral and Written Output

In some English class, students are required to repeat after the listening materials finished, which is to ask students to imitate the intonation of the learning material [20]. This method of teaching will lead to a high level of concentration and improve oral presentation skills. Besides, students are required to organize and summarize in their own words what they have heard after listening to a recording. This method can exercise students' listening comprehension skills and improve their sense of language.

The two main types of written expression are dictation and note-taking. Dictation is the most common form of written output for students. Students need to write down the whole words or sentences after listening, which can strengthen students’ retention [20]. Because of the difference from the native language, like the accent and alliteration which is hard to recognize, it is more difficult for students to identify vocabulary. But put another way, the effect of this method is very significant. Another method is note-taking. When having classes, students will write down the key words to memorize what teacher said, which avoid the strict word-by-word dictation requirement. This method improves students' ability to capture information.

4. Conclusions

This paper combines input theory with output theory, and points out the direction of English Listening Teaching in China on the basis of in-depth analysis of relevant theories. Listening is the process of processing and understanding input information, and input is the prerequisite for output. Input sufficient, comprehensible, and interesting listening materials to achieve efficient listening acquisition. However, input alone is not enough to internalize the only through output can we promote the transformation of the language input, thus forming the learner's own language system. It is only through the output that the input is transformed into the learner's own language system. Listening enables students to consolidate their knowledge of the language they have learnt and to develop their writing skills to demonstrate their language skills. They are an organic unity that affects and interacts with each other. In summary, Teachers should see listening lessons as integrated lessons, combining listening training with other language skills so that output and input activities complement and reinforce each other.

This paper only puts forward suggestions for listening teaching in China from the aspects of input and output. Due to the limited conditions, this paper does not carry out specific research on the above teaching method to test its practical feasibility. It is suggested that this method can be applied to practice in future research.