1. После used to используется чистая форма глагола (инфинитив)
Например, My sister used to HAVE long hair...
2. В отрицании used to превращается в didn't use to
Например, Her hair DIDN'T USE TO be fair and curly
3. В вопросе used to превращается в did (подлежащее) use to
Например, DID Amy USE TO have pigtails?
4. She used to have long dark hair.
When my sister was a child, she used to be rather skinny.
I used to have freckles when I was a child.
My father used to have a beard and moustache some years ago.
When Mary was a teenager, she used to be very active and curious.
When my mother was about thirty she used to have red middle-length hair
When Kate was 6 she used to wear pigtails.
She used to be plump.
I used to be short and fat when I was a child.
When he was about twenty he used to be very friendly and cheerful.