The people that already gave answers are correct. If the question is "What did you do yesterday?" the answer should be "I studied English." However, many native English speakers don't strictly follow this rule, or even know it, so you'll hear both "I was studying English" or 'I studied English." I know that I would use either of those interchangeably and not even think about whether or not it's correct.
You'll often hear the question worded both ways as well, "What did you do yesterday?" or "What were you doing yesterday?" Technically there is a difference, but a lot of people will just choose one of them without thinking about the specifics.