We are in the last couple of days of Spanish II in my 6th period and, as always, the preterite and the imperfect is the scariest part of the final exam that looms ahead in a mere matter of days. (For that matter, it might still be the scariest part of the final exam for some of my Spanish III students.)
One of the tools I use to teach the difference between preterite and imperfect are the expressions that are commonly associated with each tense. Expressions like generalmente, a veces, and siempre are usually followed by imperfect. But expressions like una vez, de repente, and en seguida are usually followed by the preterite.
I made a set of flashcards with some of the expressions. One set in English and one set in Spanish. They can use the cards to play Memory; they can draw them out of a bin and use them in a sentence; they can write the meanings on the back and study them . . . the uses are endless!
I'm sharing the file with you. You can print them on heavy card stock and cut them out, or you could distribute them to your students and have them make the flashcards. Whatever you like. I went over the expressions with them ahead of time and we discussed in the large group why each one was associated with preterite or imperfect. Then they took some notes and did some practice activities. Overall, I saw a big improvement on the last Spanish II test - so it seems to be working.
It is not perfect. I probably left off some important expressions and the ones I included might not be ones that you would choose. Additionally, I can think of some examples where a speaker might say something that did not follow the paradigm like, "Una vez yo estaba en la cocina de mi abuela cuando vi un ratón." That sentence is not incorrect - it just wasn't made for Spanish II students.
Though native speakers might not be so forgiving, I tend to be fairly direct with my final exams. If it's a preterite expression, I'll have a verb that needs preterite. I'm not going to switch it up on them and surprise them with an imperfect verb following a preterite expression - that would just be mean and wrong on so many levels. (If they go on to study the language in more depth and deal with native speakers, they will learn the exceptions to the rules then.)
I hope you find these flashcards useful and may your Monday go by happily!