This case study aimed to investigate the changes in student teachers' (STs) interactional language during their internship at local schools. The student teacher interactive language was measured using the analytical tool of improved Flanders interactive analysis system (iFIAS), and an intervention and interview were conducted afterward. The study found that the student teacher's interactive language keeps changing: the ratio of teacher language (TL) is increasing, while the ratio of student

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... uage (SL) and silence are decreasing; the teacher's direct instruction gradually shifts to indirect instruction; the number of positive integration grids (PIG) grows larger, while the number of defect grids is getting smaller; and the teacher asks more valid questions and makes more positive comments. The overall change did not come naturally, but due to the mentor's instruction, the ST's self-reflection, and the researchers' prompt feedback of the iFIAS after each lesson. The study quantified the ST's classroom language using iFIAS to provide a visualized understanding of the language. The findings can provide a reference value for future research on the interactive language of student teachers.