Form and rules
How it works
Use used to + base verb for repeated past actions: We used to play outside every day.
Use it for past states too: She used to live in Rio.
The structure normally creates a contrast between then and now.
In negatives, the most common modern form is didn't use to + base verb because did already carries the past tense.
In questions, use Did + subject + use to + base verb?: Did you use to walk to school?