The soldiers all used to do a six-month tour of duty in Northern Ireland.
士兵过去都要在北爱尔兰服役六个月。
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。
I used to do it all the time.
我以前都是这样的。