1.used to do sth.过去常常做某事,此句中的to do 为不定式,否定形式：didn't use to do sth.或 used not to do sth.如：He used to play football after school.放学后他过去常常踢足球.Did he use to play football?Yes,I did.No,I didn't.He didn't use to smoke.他过去不吸烟.
2.注意与以下短语的区别 ：1) be/get used to (doing) sth..习惯于(做)某事,此句中的to 为介词；2) be used to do sth.被用于做某事,此句中的to do 为不定式；be used for （doing）sth.被用来做某事.