It's complicated. I'm investigating it just now. Verbs don't change their form, but they use various combinations of particles (guo and le) in various different patterns to express what in English are past tenses. In chinese, it's very important to be able to specify time, so you need to concentrate a lot on placing actions in timeframes. Once you can say all your 'ago, since, for, previously, after, still, anymore' you're already at 75%.