L1 refers to the first language, or native tongue, of the students and/or teacher. (L2, or second language, can be viewed as the target/foreign language.) There are positives and negatives for use of the L1 in the class. There are also positives and negatives for avoidance of the L1 in the class. It should be understood that many institutions and private language schools prefer to institute a 100% English only rule, which fails to consider the positives of using L1 in the classroom. Each teacher should assess how to best use the L1 in his classroom, particularly how to balance teacher and student talk time. However, care should be taken if the teacher allows the students to use their first language. It's very easy for the L1 to become a crutch, which can limit the students' improvement.
As a final comment before examining use of the native language of the students in greater detail, the teacher should always consider the reasons for his use of the L1 before weighing any positives or negatives. And if the teacher wants to practice his foreign language skills, speaking the L1 of the students, then this ultimately fails to serve the class. Students should always come first.
Advantages of L1
The focus of the class often determines how much emphasis gets placed on using or limiting L1 in the classroom. A conversation class may often be better served if students try to use English as much as possible. When a student can't understand a word or phrase, or can't follow some aspect of a spoken conversation, then he has the chance to employ speaking strategies. Yet in another class which focuses on business skills, for example, then the focus may not be on English. It's on a particular skill, such as how to give presentations or conduct business with Americans. Technical or cultural explanations in the L1 of the students may be more useful in these lessons. Students practice in English with the information provided by the teacher.
When students can use their L1 to ask questions and confirm comprehension, it can lead to a clearer realization of the form and meaning of the language. Students may ask either their teacher or peers. Although students can ask similar questions solely in English, lower-level students may need further clarification on some point or aspect of the new material. They may not know how to phrase the question to the answer they seek. Without this opportunity to ask in their L1, some students may get frustrated with their inability to receive improved comprehension and language production. Some types of students need to understand the details in order to use and link the target language correctly, and so will very much need the chance to seek clarification in their L1.
...it's very easy for the L1 to become a crutch, which can limit the students' improvement
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If the teacher uses the first language of the students from time to time, the L1 can provide a change to produce sentences beyond a student's ability. Students say a sentence in their native tongue, which the teacher translates for future use and/or reference. This proves especially beneficial with incidental language, such as a singular phrase or sentence. The teacher may have otherwise ignored the opportunity, or perhaps only addressed it in the Wrap Up portion of the lesson. Students may similarly do this, as in looking at a text in their native language and translating it (or summarizing it) into the target language of the class.
The teacher might also consider using the native tongue of the class to quickly start an activity. Lengthy and complicated explanations beforehand can raise teacher talk time. It can also detract from the purpose, namely building accuracy and fluency. If the teacher wants to jump into the practice session without wasting valuable class time, a thirty-second explanation in the students' L1 may accomplish this best. An explanation in the native language of the students may be necessary with a class of false beginners where no one speaks any English at all.
It's also effective for students to provide an occasional word or sentence in their native languages, especially during activities which focus on fluency. A student may get stuck in a conversation and find themselves unable to express their opinions, answers, or questions. Most often, the student passes up the question, answer, etc. and restricts their conversation to previously studied, comfortable language. Limited approval to use their L1 allows for longer and richer discussions. However, limited and occasional use of the L1 is the key idea here. To restrict excessive use of the L1, the teacher can simply allow only two opportunities to use the native tongue per person in a five minute conversation, for example.
Disadvantages of L1
There are also disadvantages for the L1 in the class, from both the perspective of the teacher and the students.
For students, an entire class in English offers additional opportunities to hear the language. The English used for explanations and instructions represent "real" English because students actively listen how to use a grammar point or vocabulary word, or how to conduct an activity, for example. When they use the new material or complete the activity correctly, this boosts confidence. They can measure comprehension through success. (Note: Although the teacher should always strive to make activities representative of real and relevant English, students may sometimes see activities as something slightly less than real. They still question how much they've achieved. The teacher can point to explanations and instructions that were understood as proof of ability.)
Some teachers fear that tacit approval of L1 will result in its heavy use in the classroom. Students will rely on it, especially if allowed to occasionally insert a word or phrase in their native tongue. This can be true, such as when students fail to develop speaking strategies. They thus shouldn't resort to their native language immediately, but should strive to ask questions, provide explanations, or give information in English. In most classes, it doesn't take much effort for the teacher to encourage students to do so. However, it should be noted that some students may want to speak and speak and speak, resorting to their native tongue when any difficulties arise. They don't view communication as a balance between fluency and accuracy. They use the teacher's policy of occasional L1 use a little too liberally.
...the L1 can provide sentences above a student's ability
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There is also the concern that the teacher won't understand what the students are saying, assuming that he doesn't speak the same language well or at all. In student-centered classes where everyone is interested and actively engaged, though, students are focused on the target language, not the teacher. Use of the L1 among students only presents a problem when students have sought to confirm information via their peers. The explanation given between students may not be correct, which the teacher would miss. Or if the teacher opts to explain in the L1 of the students, and again is not very adept or fluent in their language, then he could easily miss much-needed nuance. He may even provide the incorrect meaning of a word or grammar structure, thereby causing greater confusion.
Lastly, it's generally agreed that the L1 shouldn't be used except in homogenous classes where everyone speaks the same native language. If the class consists mainly of Spanish students, along with a few Japanese and Chinese students, the teacher shouldn't give an explanation in Spanish. The other Japanese and Chinese students wouldn't understand.
Although using the students' first language shouldn't be considered wholly detrimental, the teacher should carefully weigh the positives and negatives of using anything other than the target language of the class.