Group work has been a main activity recommended by mathematics education reform. We aim at describing the patterns of interaction between teachers and students during group work. We ask: How do teachers scaffold group work during a problem-based lesson? We use data from a problem-based lesson taught in six geometry class periods by two teachers who did not routinely implement problem-based small group work. We applied systemic functional linguistics to examine teacher and student moves when the teachers assisted the groups. We found that the teachers' scaffolding moves exemplified analytic and social scaffolding because the teachers made the content needed for solving the problem explicit, and, also, encouraged students. The students' performance of moves showed how they controlled the timeliness and the content of the scaffolds. The findings support prior research on classroom norms that increase student opportunities for conceptual agency and illustrate how those opportunities emerge.