used to

[mas94010]

1.How does one use correctly the phrase "I used to," and what does it mean? Is this an idiom or informal English?

[Tdol]

It refers to something you did in the past, but don't do now:

When I was at school, I used to wear a uniform.

It's idiomatic, as the meaning is not straightforward, but it's prefectly acceptable in any context.