Key point
l used to collect baseball cards.我过去常常收集棒球卡。
used to do sth.
过去常常做某事(现在不做了)Eg:我过去常常骑自行车去上班。used to go to work by bike.

used to的各种句式:
否定句: sb.didn't use to do sth. 
sb. usedn't to do sth.
Eg: He didn't use to get up before 6:30.
He usedn't to get up before 6:30.

一般疑问句:Did sb.use to do sth. ?
Used sb. to do sth.?
答语:Yes, sb. did. No, sb. didn't. 
Yes,sb. used. No, sb. usedn't.