This study attempts to examine the amounts, purposes, and reasons for using L1 by teachers as well as the amounts and purposes of its use by students in English as foreign language classrooms through investigating two pre-intermediate classes of an English language institute in Iran. Among students with an age range of 16-25 years, eight male and female students were in one class and 16 were in the other class (N = 24). Two native Persian teachers with master's degrees in Teaching English as a Foreign Language participated in this study: One was 40 years old with 14 years of experience and the other one was 32 years old with nine years of experience. Eight 90-minute sessions of each teacher’s class were audiorecorded. Following that, the two teachers were interviewed to report on the reasons for which they used their first language. The findings obtained from the classroom audio transcriptions were analyzed using a combination of quantitative and qualitative approaches through the lens of Activity Theory. Moreover, the teachers’ interviews were thematically analyzed and the results of these analyses indicated that the teachers and their students resorted to the first language as an important cognitive and pedagogical tool on different occasions wherever needed during their teaching. The findings also revealed that teachers in this study maintained that using the students’ first language supports second/foreign language learning and teaching processes in the pre-intermediate levels.
