This is a large 55-question preterite vs. imperfect practice activity. I came up with 55 original sentences, all of which require students to use the preterite or imperfect tense, and organized them into a chart. For each sentence, the students must not only conjugate the verb correctly, but also provide the reason/rationale behind their choice. (The various possible reasons are listed at the top of each page.)
This is probably best suited to an honors/gifted class and takes almost an entire 50-minute class period (unless, of course, you want to assign it for homework). I also have an accompanying quiz available for purchase that follows this same format! You could have them practice with this, and then take the quiz, which is similar in style, to test their comprehension. Quiz is available here
I like to let them work at their own pace (differentiation!), then borrow a copy of the key to check their answers whenever they are finished.