1. Introduction

1.1 Definition

Code-switching occurs in the context of bilingualism and multilingualism, which refers to the phenomenon of switching between different languages or dialects in communication, that is, the speaker consciously or unconsciously chooses different languages, dialects, styles or varieties according to his or her actual needs. And it is Jakobson who first used this verb-based noun “Switching code” [1]. Starting from the 1960s, some people began to study the mechanisms by which people acquired language competence, especially the competence of learning foreign languages, hoping that some breakthrough methods could be achieved to guide the Second Language Learning. It combines linguistics, neurolinguistics, pedagogy and sociology and so on, and is slowly developing a new discipline called Second Language Acquisition. Ellis argues that second language acquisition is an unsupervised (or natural) acquisition process [2]. Additionally, the term "second language acquisition" describes the procedure for learning a second or foreign tongue. In other words, second language acquisition is a never-ending process.

1.2 Research Background

With the acceleration of global integration, language integration is inevitable. Language users, especially young Chinese, are using English more and more widely, which determines the high frequency of code-switching between Chinese and English. By reviewing a large number of literatures, it is found that scholars' researches on code-switching of second language users are mainly divided into two situations: classroom situation and non-classroom situation. The following is a summary of previous studies.

1.2.1 Application of code-switching in classroom settings

Yu Jun said that code-switching is ubiquitous in the whole process of second language acquisition, especially in teaching. Teachers often use code-switching to relieve students' learning pressure and improve their learning enthusiasm. The interpersonal and textual functions of code-switching are fully applied to teaching practice, which can more effectively help second language learners achieve the communicative function of their second language [3].

According to Yang Xiaoqiong, code-switching can be used as an effective teaching strategy and a shortcut in second language acquisition, which enables students to have immediate and direct contact with foreign language expressions, thus keeping the conversation going and understanding information accurately [4]. At the same time, the use of code-switching can also help students enhance their confidence, because they can use the conversion between their mother language and the second language to resolve their mistakes in some words or sentences, and code-switching also becomes a bridge to help them acquire the second language faster.

Yang Yan mentioned that when teaching some English difficult word and phrases, it is difficult for teachers to make students understand the exact meaning and usage of them completely if they are explained in English. Or when explaining English grammar knowledge, more professional terms will be used. If teachers use Chinese to assist their explanation, students will be able to understand more thoroughly [5].

In conclusion, as a language mechanism, code-switching can promote language development, and as a learning method, it can help students acquire a second language more efficiently.

1.2.2 The use of code-switching in non-classroom settings

With the frequent cultural and economic exchanges between China and overseas countries, more and more people are learning foreign languages, so it is inevitable that foreign languages will be reflected in the daily conversation of second language learners, which leads to code-switching.

Early studies of code-switching focused on the constraints of the macro-social environment on code-selection in communication practice. For example, Fishman studied who spoke what language to whom and when, like Ferguson’s study of Diglossia, researchers hoped to establish a correspondence relationship between language choice and social context [6,7].

In the research of Yang Xiaoqiong, code-switching has been noticed by many linguists and sociologists because of its great social significance [4]. First, code-switching can be used to fill language gaps; second, code-switching can make an individual a member of a special group; and finally, code-switching can achieve many other communicative purposes, just as monolinguals can express their attitudes and feelings in a variety of familiar ways, bilinguals can also use code-switching to achieve their goals [4] .

Clarifying the internal link between social factors and code-switching is the aim of code-switching research, so as to make better use of code-switching and achieve good information exchange. Therefore, Guo Xiaolu and Guo Hongmei showed that code-switching could be regarded as a sociolinguistic tool for bilingual or multilingual speakers from a sociopragmatic perspective [8].

In the research of WANG Yun and HU Wenjie, they argued that code-switching has important communicative functions, cultural and contemporary values. Additionally, they noted that in addition to paying attention to the significance of code-switching, people should also focus on its sensible application [9]. Positive code-switching can effectively improve the quality of interpersonal communication, promote healthy communication between people, and establish good interpersonal relationships, otherwise it will lead to poor communication, or failure in communication activities, which may affect normal interpersonal interaction. The proper use of code-switching is the basis and premise of promoting positive communication.

With an increase in cross-cultural interactions between China and other nations, code-switching has progressively spread to other languages, and it will become a communicative competence that modern people should possess. The use of code-switching in L2 learners’ daily communication has certain pragmatic and social functions. When using code-switching, it is necessary to ensure that both sides of the conversation understand the meaning and context of the code-switching language so as to make the conversation go smoothly.

1.3 Research Objectives

Code-switching is a common language phenomenon, which often appears in the daily communication of bilingual or multilingual people. It is especially used in the classroom teaching of Chinese students foreign languages.

Speakers, especially second and even third language learners, naturally use code-switching for the convenience of communication, such as the common Chinese-English code-switching word collocation, such as 做Pre, 看PPT, 作业的deadline, etc. Since these English words are not often said in Chinese or have formed fixed English expressions, learners who take English as their second language often use Chinese and English words or abbreviations to express in conversation. In addition, in the process of teaching English, in order to let students easily understand the content of the lesson, especially the unfamiliar vocabulary, long and difficult sentences, teachers use code-switching to achieve their teaching purpose. In fact, code-switching is indispensable in the primary stage of second language acquisition [3].

Therefore, code-switching has lots of functions and significance in the process of second language acquisition. In general, its main and most direct aim is to help learners acquire a second language more efficiently.

However, most of the existing researches only focus on the concept of code-switching. Researchers usually conduct research from sociolinguistics, grammar, conversation analysis and psycholinguistics, and adopt different research methods, and in the end, make some contributions to the study of code-switching in exploring the connection between code-switching and social factors, the grammar and structure of code-switching sentences, the structure of code-switching and conversation, and the psychological aspects of bilinguals when they use code-switching [10].

Looking through the previous literature, authors find that there are few studies on the combination of code-switching and second language acquisition. Since researchers have found that code-switching does play a positive role in second language acquisition, it is necessary to conduct relevant studies on how to use code switching in second language acquisition properly, so as to fill the gap in this aspect.

2 The Theories on Code-switching

Since the 1970s, code-switching has drawn significant attention in Western academia as a frequent occurrence in language contact. It can be categorized from the perspectives of grammar, sociolinguistics, psycholinguistics, and pragmatics. According to Poplack, the alternation of two languages on a grammatical level is not a random process but is instead controlled by laws like the free-morpheme constraint and the equivalence constraint [11]. Some of the syntactic characteristics of code-switching have been explained by grammatical studies of the phenomenon. Their interpretations, which are typically based on Western language systems like English, do not entirely apply to studies of Chinese-English code-switching.

Studies on code-switching from a sociolinguistic standpoint have mostly examined the connections between social characteristics such ethnic identification, background, gender, age, and socioeconomic level, as well as the social motivations for code-switching. The situational code-switching and metaphorical code-switching proposed by Gumperz and Myers-Scotton's Markedness Model are prominent among them [12,13]. This approach successfully explains the macroscopic relationship between code-switching and social factors, but does not capture the dynamic process of code-switching [14].

Psychological research focuses on the specific linguistic abilities and mental activities that motivate speakers to code-switch. Such studies are often combined with sociolinguistic perspectives, and there are relatively few related studies, among which the most influential is Giles & Clair's Accomodation Theory, which primarily explains how emotional and psychological motivation affect verbal communication tactics [15].

Although the above studies have made great contributions to code-switching, each of them is not comprehensive and systematic enough. Based on these limitations, there is a need to find a more comprehensive and convincing perspective on code-switching that can reasonably explain the influence of language, psychological cognition and social culture on code-switching. Therefore, pragmatics has become the first choice of many researchers [16]. Many scholars in China (Yu Guodong, Liu Zhengguang, He Naturally, Huang Chengfu, Li Bailey, etc.) have used Verschueren's Conformity Theory to study code-switching from the perspective of pragmatics. Verschueren argues that people are able to make appropriate choices about language because of its variability, negotiability, and adaptability. Adaptability is a response to linguistic reality, social norms, and psychological motivation [14].

The Linguistic Adaptation Theory was conceived by Verschueren in the 1980s, followed by a full-fledged theory of pragmatics research in the Understanding Pragmatics, published in 1999, a theory that is highly inclusive and explanatory [14] .

Verschueren argues that language use is a process in which language users make linguistic choices based on reasons internal or external to the language and with varying degrees of consciousness; humans are able to make linguistic choices in language use because natural language has three characteristics: variability, negotiability, and adaptability [14] . These three characteristics are the fundamental characteristics of natural human language, and they allow people to use language in a flexible way. Variability refers to the fact that there are different human languages that can be used and that these decisions are not predetermined. The ability to negotiate refers to the fact that people do not automatically choose a language; all choices are guided by highly flexible principles and strategies. Language variability implies uncertainty in language use, but uncertainty does not affect our communicative success. This is because human language has a third characteristic — adaptability; adaptability refers to the deliberative choice of language that leads to successful communication. These three characteristics of language are inseparable and are the object of study in pragmatics.

Research in the area of language use and functional analysis has sought to find a perspective and model that embraces linguistic, cognitive, social, and cultural factors, and scholars who have worked in this direction include Huang Guowen and Yu Guodong [14,17]. Code-switching is thought to have its roots in people's adaptation to successful long-term communication, according to researchers from a pragmatic perspective [14,18]. These researchers place more emphasis on the dynamic features of language, which is also the main research perspective used in this paper.

3 Empirical studies on code-switching

In the process of second language acquisition, language users are used to conveying certain meanings by using code-switching. Meanwhile, the choice of codes is based on the speaker's belief that they can “obtain benefits”. From this point of view, code-switching has certain special functions or effects in different situations, therefore, language users can choose different codes in different situations. Authors summarize the different situations in which bilinguals use code-switching in existing studies, and explore their motives, advantages and disadvantages of code-switching.

3.1 The use of code-switching in classroom settings

Code-switching often occurs when students' English level is limited, teachers need to achieve the teaching purpose: teachers can make students understand the teaching content, and through memory and training, students can master the knowledge points and use them freely [19]. Take English reading class for example, reading exercises students' understanding ability of long articles, so there will be long difficult sentences. When analyzing the text, teachers often carry out code-switching between English and Chinese. This is because the grammar itself is obscure. If it is explained in English, it is more difficult to explain clearly to students, and students with poor English level even can't understand it. The interpretation in Chinese can help students understand the grammatical more intuitively, so as to analyze the structure of the text.

For example, in this sentence: Knowing some Latin and Greek not only made thousands of English words semantically transparent to me but, as an added bonus, later I knew that I could risk using many of my Latin and Greek words in Spanish and Italian since they had already proven their transferability potential with English [20]. The vocabulary of this sentence is not difficult, but it is a long sentence with clauses and parenthesis. In order to reduce the understanding burden of students, teachers usually analyze this sentence in a mixture of Chinese and English. Especially when explaining the grammatical structure, they will use more professional terms. If teachers use Chinese to assist the explanation, it will help students understand more thoroughly.

Students use of code-switching in English classes tend to appear in when they answer teachers' questions or communicate with others, they meet some obstacles, like that they suddenly forget how to say a word in a second language, but they want to express in a hurry, then they use code-switching immediately. A more common phenomenon is that students unconsciously use code-switching when expressing in a second language, such as in the following conversation:

Teacher: What did you do on your holiday?

Student: I went to the beach and played the beach volleyball.

Teacher: Anything else?

Student:还有就是 I went to see my grandparents and had dinner together.

Students are actually thinking about the content to be said next and unconsciously insert some function words into their oral expressions. Therefore, these function words play the role of transition.

Code-switching does play a role in promoting second language acquisition. However, it should be noted that if code-switching is used too much, students will habitually use code-switching, even if they are capable of practicing second language, and thus excessively rely on their mother tongue, which will cause some obstacles to second language acquisition. Therefore, teachers should control the frequency of code-switching for students with different English levels, so as to help students acquire a second language more efficiently.

3.2 The use of code-switching in non-classroom settings

Code is a language system used by language users in communication. It can be either a language, a language variant or any other language form. In daily life or communication, people often encounter such situation: some Chinese speakers will occasionally mix some English while speaking, or Mandarin speakers will switch to a certain dialect when communicating. This phenomenon of switching from one language or language variant to another for the purpose of communication is called code-switching in linguistics.

As an important phenomenon of language communication, code-switching is very common in daily communication. Code-switching can help to communicate better with others and help the interlocutors to better understand each other, which is the communicative function of code-switching. By using code switching, speakers can better express what they really mean and avoid misunderstandings in communication. This section will analyze Chinese-English code-switching through the following examples of code-switching used by Chinese college students in interpersonal communication.

Example 1:

Amy: 我晚上能找你请教一下metaphor吗？刚从图书馆回来还没缓过来 。

Betty: 没问题，不过我现在不在宿舍，晚上回去call 你！

Amy: 嗯嗯！好！

Betty:你们宿舍有没有WIFI啊，可不可以让我连一下?

Amy: OK啊，你直接去吧，我在外面呢。

3.2.1 The efficiency of communication

Because the corresponding expressions in Chinese need to spend more vocabulary and sentences, code-switching can better achieve efficient communication. In example 1, Betty uses “WIFI” and “call” to refer to “无线局域网” and “打电话”. In fact, this type of code-switching is not only popular among Chinese college students, but also very common in other types of people’s daily communication, such as “iPhone”, “shopping” and other English words are also frequently used by Chinese people.

3.2.2 The convenience of expression

In addition, in daily communication, people will prefer simple expression. In example 1, if people communicate in pure Chinese, the conversation will turn into “你们宿舍有没有无线局域网?” and “晚上回去打电话给你”, the speaker will increase the amount of vocabulary input, the whole sentence will become more long-winded. Therefore, people frequently utilize code-switching rather than complex responses to simplify their answers in order to avoid responding to other people's questions or greetings with lengthy and complex sentences. For example, in some festivals, Chinese people will greet each other: “节日快乐！”, “身体安康！”, young people may feel that the atmosphere is not relaxed enough to greet each other, and they will use “You too” to answer. This action not only ease the atmosphere, which use English words shortly answer to friends and family to express good wishes, but also can achieve communication purposes.

Besides, some English words are difficult to translate into Chinese, or there are many kinds of translations but no unified translation. Therefore, Chinese language users, in order to avoid unclear expression, often directly use the original English words. For example, Apple Mobile’s iOS system, mobile apps and so on.

3.2.3 The needs in special occasions

In some special situations, language users choosing to use different code also stems from the influence of objective factors. The use of proper nouns and loanwords to fill in gaps in English and Chinese expressions. Examples of motivation to fill in gaps in English and Chinese in example 1 are mainly derived from academic topics. For example, metaphor, due to the lack of Chinese words in this term, most of the Chinese students in English major keep the original English expressions, which is code-switching, when they discuss problems after class. Even if there is a direct Chinese translation, the speaker will feel stiff when chatting with each other. English majors are therefore more likely to “Borrow” certain types of words to fill in the gaps between English and Chinese.

3.2.4 Close the psychological distance

It can also help us express our feelings when we communicate with others.

Example 2:

学生A: “Oh，yeah，考试通过了! ”　　学生 B: “Wow，太棒了! ”　　学生 A: “那明天一起去爬山吗?”　　学生 B: “OK! ”

Like example 2, Chinese young people often talk to their friends or classmates in this way. If the English words were deleted from a conversation, then the emotion from the original conversation could also change. So sometimes using English codes while speaking Chinese can help Chinese college students express strong emotions. Young people also use some new internet words with Chinese and English codes, such as “word妈” and “hold住”, to make the communication atmosphere relaxed and casual, close the distance between each other and increase Mutual Trust.

4 Conclusion

In the context of globalization, the use of a single language is no longer sufficient for people's communicative purposes, and thus the use of code-switching is becoming more frequent, especially among bilingual users. Nowadays, more and more teachers are using code-switching in English classes for college students to help them understand the classroom content, absorb the knowledge faster, and acquire the second language more efficiently. In addition, students are also using code-switching in their daily communication to bring them closer to each other. However, a review of a large number of studies found that the level of foreign language proficiency of college students seriously affects the frequency of code-switching. For students with low foreign language proficiency, code-switching may cause language barriers in classroom or everyday situations. Therefore, it is still a major challenge to study how to use code-switching reasonably while ensuring smooth language communication in the face of students with different levels of proficiency. But for the point of applying code-switching as a teaching strategy to instruction, it may also cause interference to second language acquisition. The conversion of phonological, grammatical, lexical, and even spelling elements from one language to another is referred to as interference. In a sense, the lack of code-switching is as important as its presence. Moreover, code-switching can help learners acquire a second language, but we must control the frequency of code-switching in order to avoid language interference during the acquisition process.