Answer:
1- Eran las dos de la mañana cuando el teléfono sonó.
2- La computadora se apagó mientras Ricardo navegaba en internet.
3- Horacio hablaba por teléfono mientras comía un sandwich.
4- De niña, Paula se lastimaba la pierna con frecuencia.
5- Ayer después de la clase, nosotros fuimos al cine.
6- Cuando Carolina era joven, leía mucho.
Step-by-step explanation:
1- It was two in the morning when the telephone ringed. In this case the verb is in the preterite (pretérito perfecto simple) because it's an action in the past which occurs in a specific moment and the action is already finished.
2- The computer turned off while Richard was surfing the web. Here the action is in the imperfect form because Richard was doing something when he got interrupted.
3- Horacio was talking on the phone while eating a sandwich. In this case, the action is in the imperfect because there are two actions taking place at the same time, both in the past.
4- As a kid, Paula used to hurt her leg frequently. The imperfect replaces what in English is the "used to" form. That is to say, is something which was usual in the past. In this case, what was usual was Paula getting her leg hurt.
5- Yesterday after school we went to the movies. Here the action is in the perfect form as it's something that begins and finishes in the past in a specific moment.
6- When Carolina was young, he read a lot. The action is in the imperfect because it's not a specific moment in the past. The time itself is not that important. It was during all her young years that she read, that's why the past form is imperfect.