is the english translation correct? Questo succedeva ogni inverno is "this happened every winter." I think "this would happen every winter" is questo succederebbe ogni inverno.

"Questa succedeva ogni inverno" doesn't work. Is the masculine form the default?

Questa or questo should work here because there is no indication of gender.

Is "Questo accadrebbe ogni inverno" really wrong?

This HAPPENED every winter and this WOULD HAPPEN every winter is not quite the same thing. I'm thinking ponji1 is correct.

I think "Succedeva tutti gli inverni" should be accepted. The meaning is obviously the same, and the phrases are quite interchangeable in daily speaking.