The purpose of this study was to examine how the use of the student's first language (L1) by a non-native English-speaking EFL teacher affects the students' experiences in learning English compared to those in the classrooms where only English is used as a means of teaching. This study also investigates the role of the teacher in providing comprehensible inputs to her EFL learners. A total of four groups participated in this study (approximately 25 students per group). Two groups were randomly assigned to be taught by using both Thai and English languages while the other two groups were taught with English only. Two qualitative data collection methods employed included focus group interviews and classroom observations. A Comprehensible Inputs Checklist was developed to code the classroom observation data and the results were analyzed using a cross case analysis technique. From the cross-case analysis, it was revealed that the students' English competency level and their attitudes toward English played the most important role in the students' learning of English. The teacher's use and non use of the L1 would have a significant effect only when it came to difficult topics or topics that are not related to the students' backgrounds and interests. The results from the checklist showed that the teacher supported and provided "enough" comprehensible inputs during her teaching. Surprisingly, instead of benefiting more on the students' learning, the use of the L1 was found to help facilitate the teaching of the non-native EFL teacher. Furthermore, classroom atmosphere, as well as students' interactions in the participating classrooms, were analyzed and discussed. (Contains 2 tables.)