過去式(Preterite &Imperfect)

The preterite is used:

To tell of something that happened once —
Fui ayer a la tienda. (I went to the store yesterday.)
Escribí la carta. (I wrote the letter.)

To tell of something that happened more than once but with a specific end —
Fui ayer a la tienda seis veces. (I went to the store six times yesterday.)
Leyó el libro cinco veces. (He read the book five times.)

To indicate the beginning or end of a process —
Tuvo frío. (He got cold.)
El huracán se terminó a las ocho. (The hurricane was over at 8.)

The imperfect indicative is used:

To tell of past habitual or repeated actions —
Iba a la tienda. (I used to go to the store.)
Leíamos los libros. (We would read the books.)
Lavaban los manos. (They would wash their hands.)
Escribía muchas cartas. (I wrote many letters.)

To describe a condition, mental state, or state of being from the past —
Había una casa aquí. (There used to be a house here.)
Era estúpido. (He was stupid.)
No te conocía. (I didn't know you.)
Quería estar feliz. (He wanted to be happy.)
Tenía frío. (He was cold.)

To describe an action that occurred over an unspecified time —
Lavaban los manos. (They were washing their hands.)
Cuando José tocaba el piano, María comía. (While José was playing the piano, María was eating.)

To indicate time or age in the past —
Era la una de la tarde. (It was 1 p.m.)
Tenía 43 años. (She was 43 years old.)