1. Preterito vs. imperfecto
Repaso D

2. Both the preterite and the imperfect are used to talk about things that happened in the past.

3. The choice of which to use depends on if you are describing an completed action or a continuous, recurring action in the past.

4. You use the preterit to express actions or events that began and ended at a specific time in the past.
Ejemplo: Anoche volví a casa.
Last night, I returned home.

5. You use the imperfect to talk about a continuous or repeated action in the past. When the action began or ended is unimportant.
Ejemplo: Cada noche volvía a casa.
Every night, I returned home.

6. Dos acciones pasadas en la misma frase
Often a sentence may have two or more verbs in the past. The verbs may be in the same tense or in different tenses.

7. Dos acciones pasadas en la misma frase
1) In the example, both verbs are in the preterite. Both describe simple actions that began and ended at a specific time in the past.
Laura llegó ayer y Pepe la vio.
Laura arrived yesterday, and Pepe saw her.

8. Dos acciones pasadas en la misma frase
2) In the example, both verbs are in the imperfect because they describe habitual/continuous actions. When the actions began or ended is unimportant.
Durante el invierno, Adela iba a las montañas a esquiar, pero yo trabajaba.
During the winter, Adela went to the mountains to ski, but I worked.

9. Dos acciones pasadas en la misma frase
3) In the example, the verb estudiaba is in the imperfect; it describes the background—what was going on. The verb entró is in the preterite; it describes the action or event that interrupted the ongoing action.
Yo estudiaba cuando Julia entró.
I was studying when Julia entered.