Unlike English, Spanish has two simple past tenses, known as the preterite (often called the preterit) and the imperfect indicative. (As in English, they are known as simple tenses to distinguish them from verb forms that use an auxiliary verb, such as "has left" in English and ha salido in Spanish.)
Although the English simple past in a sentence such as "he ate" can be conveyed in Spanish using either the preterite (comió) or the imperfect indicative (comía), the two tenses are not interchangeable. In general, the preterite is used when speaking of completed action, that is, when the verb refers to an action that has a clear end. On the other hand, the other past tense is known as an imperfect tense because "imperfect" can also mean "incomplete"; the imperfect indicative is used to refer to an action that doesn't have a specific ending. (At this point, it isn't important to know the meaning of "indicative," except that it contrasts with "subjunctive," a verb type that usually isn't studied by beginning students.) Following are some more specific uses that should clarify the difference; note that the imperfect is frequently translated in ways other than the English simple past.
The preterite is used:
To tell of something that happened once — Fuimos ayer a la playa. (We went to the beach 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 las 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.)
Background — The imperfect indicative is frequently used to provide the background for an event that is described using the preterite. Era [imperfect] la una de la tarde cuando comió [preterite]. (It was 1 p.m. when she ate.) Yo escribía [imperfect] cuando llegaste [preterite]. (I was writing when you arrived.)
Differences in translated meaning — Because of the way the two tenses are used, some verbs can be translated using differing words in English depending on the tense in Spanish. This is especially true when the preterite is used to indicate the beginning or end of a process. Conocí al presidente. (I met the president.) Conocía al presidente. (I knew the president.) Tuvo frío. (He got cold.) Tenía frío. (He was cold.) Supe escuchar. (I found out how to listen.) Sabía escuchar. (I knew how to listen.)
Some of the sentences on this page could be stated in the other tense with a change of meaning. For example, while "escribía muchas cartas" would be the typical way of saying "I wrote many letters," as that is something that typically would take place over an unspecified period of time, one also might say "escribí muchas cartas." But the meaning of the sentence, not readily translatable without a context to English, would change to indicate that the speaker was referring to a specific point in time. For example, if you were talking about writing many letters while you were on a particular trip, you might use the preterite form.