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.
写出过去进行时的标志词和标志句子,他的构成方式是怎样的,至少写出3中标志词及短语.各造两个句子