1) 过去进行时表示过去某段时间内持续进行的动作或者事情。
We were watching TV from seven to nine last night。
2) 过去进行时可以表示在过去某个时间点发生的事情。
What was she doing at nine o‘clock yesterday?
She was doing her homework then。
When I saw him he was decorating his room。
当我看见他的时候他正在装饰房间。(when从句表示时间点)
3) 在复合句中,如果主要动作和背景动作都是延续的或同时发生的,那么主从句的动词都可用过去进行时。
When he was waiting for the bus, he was reading a newspaper。
He was cleaning his car while I was cooking。