We use 'used to' and 'didn't use to' to speak about past habits and states that are not the same in the present.
To describe habitual, repeated actions in the past, or past facts, we generally use the construction "used to + verb." Used to is only used in positive statements (with a positive verb).
Important: Used to is used to talk about past states that are not true in the present.
When I was little, we used to go camping a lot.
When my father was young, he used to be a lifeguard.
I used to work in London, but now I work here, in Canterbury.
To describe things which were not true in the past but are true in the present, we use 'didn't use to + verb'. The meaning with this form is that something was not true in the past, but that it is true in the present.
I didn't use to like dogs when I was a boy.
I didn't use to enjoy working here.
To ask questions about things in the past which we are sure are not true in the present, we use 'Did + subject + use to + verb...?'
Did you use to go to school in London? (You don't go to school now...)
Did you use to have a girlfriend called Sophia? (I know you don't now...)
In spoken English, people often use the the modal "would," followed by the main verb to talk about past habits:
When we were kids, we would play football all the time.
When I was little, we would go camping a lot.
When my father was in school, they would cane children who didn't behave.