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 didnt turn up.
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