過去進行時的用法詳解

　　(1) 表示在過去某一時間正在進行的動作，此時句中往往有表示過去的時間狀語then,at that time,this time,yesterday等。

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

　　(2)敘述在過去的同一時間都在進行的幾個動作，通常用While

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

　　(3)表示在過去某一段時間內一直持續進行的動作

　　They were expecting you yesterday, but you didnt turn up.