This study investigates how shared understanding is established during a rarely researched instructional and interactional context, namely teacher-student interactions during between-desk instructions (BDIs). Specifically, the authors focus on instructional interactions initiated by students during project-based language teaching in an upper-secondary Swedish classroom and probe into how students' formulations of understanding of a teacher's prior responses shape the subsequent interactional trajectories. Their conversation analytic investigation reveals that the teacher produces either confirming or disconfirming actions following students' formulations of understanding. These response types accomplish two distinct forms of interactional work: (a) when the teacher confirms the students' formulations, she expands the sequence with instruction-related elaborations; and (b) when the teacher does not confirm the formulations, she accounts for that, prompting students to reformulate their understanding. Overall, this study contributes to the body of research on BDIs as a recurring yet under-investigated lesson practice during project work.