I use this PowerPoint to review the uses of the preterit and the imperfect. I create a class set of mini-whiteboards for students by putting cardstock in plastic sheet protectors. I cut pieces of felt into small squares for erasers. Students use whiteboard markers to practice conjugating the verbs in either the preterit or the imperfect as needed. Students show their answers and I choose students at random to tell me the reason the preterit or the imperfect is needed.
Sometimes I use the PowerPoint as the questions for a game. Students love this!