Past Habits & States (Something you did before)Use "used to + base verb" to describe a repeated action or a long-lasting situation that happened regularly in the past but is no longer true today
Rule: Subject + used to + verbExample: I used to play the piano. (I did this often in the past, but I don't play anymore).
The Negative and Question Rule (Did/Didn't)This is the most common mistake in English grammar. Because "used to" is a past tense form, you only use the -d at the end for positive statements. When you ask a question or make a negative sentence, the helper verb did / didn't so it drops down to "use to" (without the "d").
Negative: Subject + didn't use to + verb
Example: I didn't use to like coffee. (NOT I didn't used to like coffee)
Question: Did + subject + use to + verb?
Example: Where did you use to work? (NOT Where did you used to work?)