It is better not to use "used to" in questions or negative forms; however, this is sometimes done in informal spoken English. It is better to ask questions and create negative sentences using Simple Past.
USE 1 Habit in the Past
"Used to" expresses the idea that something was an old habit that stopped in the past. It indicates that something was often repeated in the past, but it is not usually done now.
Jerry used to study English.
Sam and Mary used to go to Mexico in the summer.
I used to start work at 9 o'clock.
Christine used to eat meat, but now she is a vegetarian.
USE 2 Past Facts and Generalizations
"Used to" can also be used to talk about past facts or generalizations which are no longer true.
I used to live in Paris.
Sarah used to be fat, but now she is thin.
George used to be the best student in class, but now Lena is the best.
Oranges used to cost very little in Florida, but now they are quite expensive.
"Used to" vs. Simple Past
Both Simple Past and "Used to" can be used to describe past habits, past facts and past generalizations; however, "used to" is preferred when emphasizing these forms of past repetition in positive sentences. On the other hand, when asking questions or making negative sentences, Simple Past is preferred.