POSITIVE(AFFIRMATIVE): SUBJECT+USED TO+V1
I USED TO SMOKE WHEN I WAS A TEENAGER.
NEGATIVE: SUBJECT+DIDN’T(DID NOT) USE TO+V1
I DIDN’T(DID NOT) USE TO SMOKE WHEN I WAS A TEENAGER.
QUESTION(INTERROGATIVE): DID+SUBJECT+USE TO+V1
DID I USE TO SMOKE WHEN I WAS A TEENAGER?
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
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
I used to live in
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.
NOTE : 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.