过去进行式 (was/were + V-ing)
过去某一动作发生时(V-ed)正在进行的另一动作(was/were + V-ing)
What were you doing when he came? 他来的时候你在做什么？
I was writing a report when he came. 他来的时候我正在写报告。
What were they doing when the earthquake happened?
They were watching TV when the earthquake happened.
My mother cut her finger while she was cooking.
I was taking a nap when you called yesterday.