过去进行时多用在when引起的从句中，表示某事发生时某个动作正在进行。

常有的标志词有：at that time e.g:When my father came back I was doing my homework.也表示过去的某个时间正在进行的动作。

标志词有：at 8:00am yesterday     at this time last night  其结构为：助动词"was/werw+doing"，它的概念是：表示过去某一时刻正在进行的动作或存在的状态。

e.g:My grandpa was taking exercise for his healthat 8:00am yesterday.     His family was watching at this time last night.。