過去進行時的主要用法
1.表示過去某一時間內(nèi)(正在)進行的動作
　如：I was doing my homework at 9 p.m. last Sunday.
2.表示一動作正在進行時，另一動作同時正在進行.
　(1)用while連接(while只接doing)
　例:My mother was cooking while I was doing my homework.(可以持續(xù)動作時態(tài)一致)
　(2)when表示(when可以接doing或did，when后動作時間長，用doing，時間短，用did)
　例:I was doing my homework when my mother came in.(間接表達了具體時間)
3.重復(fù)的動作
　When he worked here, John was always making mistakes.
4.表示過去將要發(fā)生的動作
　She told us Mr.Lee was arriving here.
5.用于故事，以提供其時間背景
6.表示禮貌