A: I was waiting for you at the school gate at half past seven yesterday morning. Where were you?
B: I was already in my classroom.
A: Did you see Mary?
B: Yes. I was talking to a new classmate when Mary came in.
A: Did she jion you?
B: No. She was looking at the time table on the wall while we were chatting.
说明：过去进行时表示过去某一时刻或某一阶段正在进行的动作。这里所讲的某一时刻或某一阶段可以用具体的时间（如at half past seven yesterday morning）或一个时间状语从句（如when Mary came in）来表示。