Any student who is learning Spanish has some trouble with learning how to navigate the waters of Preterite vs. Imperfect. The mistakes teachers make are in loading students down with rules to follow or having students practice fill in the blank worksheets because neither is from the angle of being the storyteller.
This handout is different from most because it is written from the point of view of someone telling a story, so it puts your students in control of the verbs they need to use to express themselves. I wrote this as if I was one of them. The questions are things I would ask myself before using verbs to speak in Spanish, and they really helped my accuracy!