Used to is used to talk about past habits or things that do not happen any more. It has the same form in all the persons, singular and plural. It is followed by the infinitive.
Christina used to eat meat and sausages (= Christina doesn't eat them any more because she's now a vegetarian)
I used to do karate when I was a teenager but I don't do it now
We form questions and negations with the auxiliary verb did/did not (didn't), the subject and the verb "use" without -d.
Did Christina use to eat meat?
George didn't use to work so hard
|Did you.....?||Yes, I/We did||No, I/we didn't|
|Did he /she/it ...?||Yes, he/she/it did||No, he/she/it didn't|
|Did they...?||yes, they did||No, they didn't|