Amina used to live near the harbor. (now she lives downtown)
We used to walk to school every day.
Not "I use to play tennis." — say "I used to play tennis.". 这里有 -d：used to。意思是你以前做过，但现在不做了。
试一试: Amina ___ live near the harbor, but now she lives downtown. (used to / use to / uses to) 答案: used to — 某件事以前是真的、后来变了，用 used to，带 -d。"Use to"（没有 -d）和 "uses to" 在这里都不是正确的过去形式：she used to live there。
didn't use to
要说某件事以前不是真的，就说 didn't use to + 动词原形。didn't 后面要去掉 -d：是 use to，不是 "used to"。
I didn't use to like coffee. (now I do)
She didn't use to study hard.
We didn't use to have a car.
Not "I didn't used to like coffee." — say "I didn't use to like coffee.". didn't 后面没有 -d：didn't use to。
试一试: I ___ like coffee, but now I drink it every day. (didn't use to / didn't used to / don't use to) 答案: didn't use to — 这句话用词基本对，但 didn't 后面不能有 -d：didn't use to（不是 "didn't used to"）。而且说过去的事要用 didn't，不是 "don't"。
Did you use to ...?
要提问，就用 Did + 这个人 + use to + 动词原形开头。同样，Did 后面是 use to，没有 -d。
Did you use to play tennis?
Did she use to live here?
Where did you use to work?
试一试: ___ you use to play tennis? (Did / Do / Were) 答案: Did — 问过去的事要用 Did 开头：Did you use to play tennis? "Do" 问的是现在的事，"Were" 问的是某人过去是什么样子，不是过去做过什么。
试一试: She ___ smoke, but she ___ exercise much — she never went to the gym back then. (used to … didn't use to / use to … didn't used to / used to … didn't used to) 答案: used to … didn't use to — 单独用的时候保留 -d：used to smoke。但跟 didn't 放在一起，-d 就没了：didn't use to。普通句子里有 -d，跟着 did/didn't 就没有了。