We’ve seen how that Spanish unlike English uses two past tense. The Indefinido for single events in the past that have been considered completed at a specific time, whereas the Imperfecto is used to describe events in the past in a ‘continuous‘ sense, that is to speak of things that were ‘happening‘ or that we ‘used to’ do.
Think of our Timeline here with the Indefinido representing the X’s, the single events, that occurred on the timeline and the Imperfect as being the continuous line that runs the length of the timeline, with no specific time being referred to.
Click the following links (in order) to review the uses of each and then practice them:
Overview from spanishdict.com with associated practice quizzes.
An excellent series of exercises from BK Nelson that will get you into the habit of using both tenses.