This article explores the factors that affect the extent of students' English language use in English-medium instruction (EMI) science classrooms. Semi-structured interviews were conducted with seven science teachers from private and public schools in Astana city, and thematic analysis was employed to analyze the qualitative data. The findings from the study highlight several factors that influence the extent of English language usage in science subjects, focusing specifically on the teacher factor. The teacher's language skills and methods affect students' English usage, with negative factors including Kazakh/Russian language use. The initial level of English is also important, such as the number of years of English language training and preparation for international exams. Understanding these findings can help inform strategies and interventions aimed at promoting and enhancing students' English language usage in science subjects, ultimately improving their comprehension and academic performance.