Feeling stuck on perfective vs imperfective aspect—any hacks?

I have 20 minutes a day to study, and the hardest part of learning Greek is definitely the aspect system; I constantly trip over which verb form to pick for 'I was doing' versus 'I did.' Since I don't have much time for deep study, is there a simple way to practice this on Chickytutor.com to get instant corrections and build muscle memory for these verb patterns?