Sinclair and Coulthard, and later Mehan, observed that classroom talk is dominated by a recurring three-move exchange: the teacher's initiation (typically a question or prompt), the student's response, and the teacher's feedback (evaluation or follow-up on that response). This IRF (also called IRE, for Initiation-Response- Evaluation) sequence is the basic building block of teacher-led lessons.
This compact scheme is the standard instrument for annotating the turn-by-turn structure of instructional dialogue. Tag each move with its function in the exchange. The distinguishing test is not who is speaking but what the turn does: an initiation opens a slot for a reply, a response fills it, and feedback closes the exchange by evaluating or taking up what was said. Ordinary conversational turns that do not form part of a teaching exchange are left untagged.