When do you use the preterite and when the imperfect?
The preterite reports a completed event; the imperfect describes the situation around it. If you are moving the story forward — this happened, then this happened — use the preterite. If you are setting the scene, describing a habit, or saying what was going on when something else interrupted, use the imperfect. English collapses both into one past tense, which is why the distinction has to be learned rather than translated.
Events against background
The clearest way to think about it: the preterite is the plot and the imperfect is the set. In any narrative, the imperfect tells you what the world was like and the preterite tells you what happened in it. A story built entirely from one or the other reads as broken to a Spanish speaker, even if every individual verb is defensible.
Llovía cuando salí.
It was raining when I left.
Background: llovía. Event: salí.
Estudiaba cuando sonó el teléfono.
I was studying when the phone rang.
Fui al mercado y compré pan.
I went to the market and bought bread.
Two completed events — the story moves.
Habits take the imperfect
Anything that used to happen repeatedly is imperfect, and this maps neatly onto English used to and would. If you can insert used to without changing the meaning, you want the imperfect.
De niño jugaba al fútbol.
As a child I used to play football.
Siempre comíamos juntos.
We always ate together.
A few verbs change meaning
With certain verbs the two tenses translate differently, because the preterite marks the moment the state began rather than the state itself. These are worth memorising as pairs — they come up constantly and the difference is not subtle.
Sabía / Supe
I knew / I found out
Conocía / Conocí
I knew (a person) / I met
Quería / Quise
I wanted / I tried
No quería / No quise
I didn't want to / I refused
Common mistakes
Ayer estudiaba tres horas.
Ayer estudié tres horas.
A bounded stretch of time — three hours, finished — is a completed event, so preterite.
Cuando era joven, fui a la playa cada verano.
Cuando era joven, iba a la playa cada verano.
Cada verano marks a habit, which takes the imperfect.
Conocía a mi mujer en 2010.
Conocí a mi mujer en 2010.
Conocer in the preterite means met. The imperfect would mean you already knew her throughout 2010.
Frequently asked questions
Is there a trick for choosing quickly?
Ask whether you are saying what happened or what things were like. What happened is preterite; what things were like is imperfect. For the harder cases, check whether the action has a clear boundary — a start, an end, or a stated duration — because a boundary usually means preterite.
Can both tenses appear in one sentence?
Constantly, and that is the normal pattern. Estaba leyendo cuando llegaste puts the background in the imperfect and the interrupting event in the preterite. If a sentence has an interruption in it, you almost certainly need both.
What about the perfect tense — he hablado?
That is a third option and in Spain it is used heavily for the recent past: hoy he comido rather than hoy comí. Latin American Spanish uses the preterite in many of those places. Both are correct in their own regions.