1、肯定句：He used to go to work on foot.
否定句：He didn't use to go to work on foot.
一般疑问句：Did he use to go to work on foot?
肯定回答：Yes, he used to.
否定回答：No, he didn't use to.
2、肯定句：They used to live here.
否定句：They didn't use to live here.
一般疑问句：Did they use to live here?
肯定回答：Yes, they did.
否定回答：No, they didn't.
3、肯定句：I used to do exercise.
否定句：I didn't used to do exercise.
一般疑问句：Did you used to do exercise?
肯定回答：Yes, I do.
否定回答：No, I don't.
used to用法详解
1、其基本义为“过去常常”，注意以下用法：
（1）一般不能与表示次数（如twice，three times等）或一段时间（如three months，five years等）的词语连用，但可与always，often，sometimes，never等频度副词连用。
（2）构成否定式和疑问式时有两种形式：借助助动词did（较口语化）或直接将used用作助动词（较正式）。
2、used to与would的区别
（1）两者均可表示过去习惯性的动作，有时可互换。如：
When we were children we would go skating every winter. 我们小时候，每年冬天都去溜冰。
（2）当要表示过去持续的状态或表示过去与现在的对比（即“以前这样，但现在不这样了”）时，则要用used to。