To describe an ability that someone had in the past, use the phrase "___ used to be able to ___".

Didn't you use to be able to do a back handspring?

In native-sounding English, it's much more common to use "used to be able to" than "could". We use "could" mostly to talk about something that a person was able to do one time in the past, like this:

I could see them, but I couldn't hear what they were saying.