如下：
1、used to+动词原形，表过去经常但现在已不再维持的习惯动作。
He used to smoke.
他过去抽烟。
2、be/become/get used to+动名词doing，表习惯于。
He has got used to getting up early.
他已经习惯了早起。
3、过去进行时，表示过去某个时间点或某段时间内正在发生的动作。
The boy was doing his homework when his father came back from work.
男孩正在做他的家庭作业当他父亲下班回来。
4、主句（had done）, before + 过去时间状语。
He had finished his homework by 9 o'clock last night.
昨晚9点钟他就做完作业了。
