This powerpoint presents the difference between the preterite and imperfect tenses in Spanish. It covers many different ways of thinking about the difference (frankly, more than can be presented in a class period, so you will need to choose which slides to skip).
It then ends with scaffolded practice helping students identify which tense they would use, then asking them to conjugate verbs using context clues to determine the tense, then gives them an opportunity to make their own sentences.
You may also be interested in getting my worksheet with 20 different sentences that students must complete using either preterite or imperfect, based on context and the key phrases they see. It's a great way to check that they understood when to use which.
OR, SAVE even MORE by getting both all the preterite AND imperfect presentations together, including the 20 sentence practice on preterite vs. imperfect. (click here for info on that)