Used to

Usually when we speak about past time we use different forms of Past tenses such as Past Simple, Past Continuous, Past Perfect etc. However, there is one more way to express something happened in the past - it's construction used to + Verb.

We use this expression to talk about habits or repeated actions in the past which we don't do in the present. We also use it to talk about states in the past which are no longer true. For example:

I used to have long hair (but now I have short hair).
He used to smoke (but now he doesn't smoke).
They used to live in India (but now they live in Germany).

The main difference between Past Simple and the construction used to is that Past Simple is used mainly when we speak about some facts and actions that happened at exact time or period of time (for Past Continuous) and used to is used to highlight the contrast between past and present.

The negative form is:

I didn't use to have long hair (but now I have long hair).
He didn't use to smoke (but now he smokes).
They didn't use to live in India (but now they live in India ).

And questions:

Did you use to be a teacher?
Did he use to study French?

Examples are taken from https://www.perfect-english-grammar.com/used-to.html