过去进行时的主要用法
1.表示过去某一时间内(正在)进行的动作
　如：I was doing my homework at 9 p.m. last Sunday.
2.表示一动作正在进行时，另一动作同时正在进行.
　(1)用while连接(while只接doing)
　例:My mother was cooking while I was doing my homework.(可以持续动作时态一致)
　(2)when表示(when可以接doing或did，when后动作时间长，用doing，时间短，用did)
　例:I was doing my homework when my mother came in.(间接表达了具体时间)
3.重复的动作
　When he worked here, John was always making mistakes.
4.表示过去将要发生的动作
　She told us Mr.Lee was arriving here.
5.用于故事，以提供其时间背景
6.表示礼貌