1、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。
2、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。
3、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。
4、e.g:My grandpa was taking exercise for his healthat 8:00am yesterday. His family was watching at this time last night.
