老师正在黑板上讲解课程。

The teacher is ___explaining the lesson on the board.

老师正在解释为什么今天没有家庭作业。

The teacher is ___explaining why there is no homework today.

老师在黑板上画图来讲解这个数学概念。

The teacher is ___explaining the math concept by drawing diagrams on the board.