过去进行时的用法详解

　　(1)    表示在过去某一时间正在进行的动作，此时句中往往有表示过去的时间状语then,at that time,this time,yesterday等。

　　I was doing my homework when my mother came to home.

　　(2)叙述在过去的同一时间都在进行的几个动作，通常用While

　　I was studying at college while my brother was teaching at university..

　　(3)表示在过去某一段时间内一直持续进行的动作

　　They were expecting you yesterday, but you didn't turn up.