used to do过去常常，过去曾经；过去常常做某事；意思是现在不做了，主语常是人，当然也可以是能实施动作的动物等。
I used to do that when you were a little girl, remember?
在你还是小女孩的时候我曾经那么做过，记得吗？
Potential energy can be used to do work.
势能可以用来做功。
The lecturer used to do well, but he seems to have gone off now.
这个讲师以前挺好，但如今似乎讲得差了。
