Todos los veranos, yo (1) [iba] (ir) a la casa de mis tíos en el campo. Mis primos y yo (2) [hacíamos] (hacer) muchas cosas. Durante el día, (3) [íbamos] (ir) a la ciudad. Por las tardes, mientras Benito (4) [tocaba] (tocar) la guitarra, nosotros (5) [cantábamos] (cantar). Todos los sábados, (6) [salíamos] (salir) de excursión a las montañas. Allí siempre (7) [jugábamos] (jugar) y (8) [nadábamos] (nadar) en el río. Un día, mi prima Isabel (9) [oyó] (oír) un ruido que (10) [venía] (venir) de una montaña. De pronto, todos (11) [corrimos] (correr) para ver qué (12) [había] (haber) allá. Al llegar al lugar (13) [descubrimos] (descubrir) un salto. Nosotros (14) [tomamos] (tomar) muchas fotos y luego (15) [regresamos] (regresar) a la casa. (16) [Nos divertimos] (divertirse) mucho ese día. Yo nunca voy a olvidarlo.

1. Imp: The key phrase here is "todos los veranos." This is a regularly occurring event in the past.
2. Imp: They regularly did many things during that time period, i.e. during their youth. The phrase "todos los veranos," was mentioned in the previous sentence, so this idea still applies.
3. Imp: They regularly went to the city.
4. Imp: While benito (regularly) would play the guitar,...
5. Imp: We would sing. Notice that in 4 and 5 they are doing these two things alongside one another. One action is not interrupting the other one.
6. Imp: The key phrase here is, "Todos los sábados." This is a regular occurrence of what they used to do.
7. Imp: This is a regular occurrence of what they used to do "Todos los sábados."
8. Imp: This is a regular occurrence of what they used to do "Todos los sábados."
9. Pret: The key phrase here is "un día." This marks a shift in the story. Now it is no longer talking about what used to happen regularly, but what happened on one particular day.
10. Imp: She heard a sound "which was coming from a mountain." We later find out that it is a waterfall which is an ongoing sound. Without reading ahead, we would not know whether to put preterite or imperfect here.
11. Pret: "De pronto" is the key phrase. They run once.
12. Imp: What was going on there. The waterfall would be an ongoing sound and not a one-time sound.
13. Pret: They discover it once. It occurs once.
14. Pret: Numbers 14 and 15 list a series of actions. When this happens, the preterite is used unless it is explicitly talking about habitual past actions.
15. Pret: (This applies to 14 and 15.) The preterite tends to move the story along.
16. Pret: This is a summary of the day. It therefore uses the simple past, i.e. the preterite.