This handout is meant to save you time! Just make copies and explain to your students that the preterit is used for completed actions and the imperfect is used for on-going actions or descriptions in the past. WS 400 has the uses of the preterit and the imperfect listed side by side for easy reference. It also includes the verbs that change meaning in the preterit and the imperfect as well as the key words that signal the use of the preterit or the imperfect.
In my classes, I distribute WS 400 and explain the uses, then I create mixed-ability groups of 4 and students work on WS 401 ID Preterit and Imperfect together. Students are responsible for identifying whether the preterit or the imperfect is needed and explaining why that tense is needed. In groups of 4 they have 3 other students to help them understand why one tense is needed over the other.
I hope this handout shortens your prep time!--SraBernal