When should you use the preterite or the imperfect to express past time?

Question

_{This is a canonical question / Esta es una pregunta canónica}

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There are two ways to express simple past time actions and conditions in Spanish. One is the preterite,

Comí tacos. _{I ate tacos.}

Besé a una chica. _{I kissed a girl.}

and the other is the imperfect,

Comía tacos. _{I ate tacos.}

Besaba a una chica. _{I kissed a girl.}

and they are not interchangeable.

Sometimes the Spanish imperfect is compared to the English imperfect ("I was eating tacos.") or the used to paraphrase ("I used to kiss a girl."), but there are many cases where those don't mean the same thing as each other and even more where they don't mean the same thing as the imperfect in Spanish.

One of the differences is in indirect speech (see Does indirect speech in Spanish require changes in tense, mood, etc?) but there are other subtleties. How do you keep track of them and make them feel natural?

Answer 1

You use the preterite for an action that happened at a distinct point in time. So to expand on your examples:

Comí tacos ayer.

I ate tacos yesterday.

Besé a una chica en la fiesta anoche.

I kissed a girl at the party last night.

These were both specific points in time.

The imperfect is used for actions that happened over a period of time, and it is also commonly used in place of estaba + gerund.

So:

Cuando era joven, comía tacos.

When I was young, I would eat tacos. / When I was young, I used to eat tacos.

Besaba a una chica durante la película.

I was kissing a girl during the movie.

Notice my first example; this is a very common pattern in Spanish in which both tenses are used. The imperfect is used for the ongoing action that was occurring when the other one interrupted or occurred. For example,

Lavaba los platos cuando el teléfono sonó.

I was washing the dishes when the phone rang.

And, as mentioned, estaba lavando could also have been used there, but it's common to say it the way I did as well.

Basically, if a distinct point of time is mentioned (ayer is usually distinct, cuando era niño is usually not) and the action was not ongoing in the context of the sentence, use the preterite. Otherwise, use the imperfect. Note that cuando era niño could also require the preterite if you are talking about something that happened once, e.g.

Cuando era niño, fumé una vez con mis amigos.

When I was little, I smoked with my friends once.