- Distinguish the use of imperfect for ongoing actions and preterit for interrupting actions in the past.
Depending on how you combine the preterit and the imperfect together you are expressing a different meaning, because the tenses themselves have different relations to time. In English, these distinctions have to be expressed by means of adverbs or prepositional phrases, but in Spanish the choice of preterit or imperfect expresses not just that the action happened in the past, but also *how* the action happened.
- A combination of imperfect with imperfect:
When you want to convey that two actions were ongoing at the same time in the past, you use the imperfect tense. You will often see the words “mientras” (while) and “y” to show that these actions were in progress simultaneously.
- A combination of preterit with preterit:
When you want to convey a series of completed actions in the past, you use the preterit tense for each of them. It is very important that the events are stated in chronological order.
- Mi tía se levantó a bailar, el tacón de su zapato se rompió y ella se cayó. (My aunt got up to dance, the heel of her shoe broke off, and she fell.)
- Mi tío la salvó antes de tocar el piso, la levantó y la hizo girar por un momento. (My uncle caught her before she touched the floor, he lifted her up and twirled her for a moment.)
- A combination of imperfect with preterit:
When you want to show that an action was in progress when another action began, you use the imperfect for the continuous action and the preterit for the interrupting action.