preterite or imperfect? the question that decides it

Most people are taught this as a rule about time: the preterite for things that happened once, the imperfect for things that went on for ages. It works often enough to get through a lesson and then falls over in an exam, because Spanish is not asking how long anything took.

It is asking one thing: is this action finished, or is it the background?

the question to ask

Every time you have to choose, ask whether the verb is describing a thing that happened or what was going on at the time.

• A thing that happened, with a beginning and an end: preterite. Fui al cine — I went to the cinema. It happened, it is over, next.
• What was already going on, or what used to happen: imperfect. Iba al cine todos los sábados — I used to go to the cinema every Saturday.

The length of time is irrelevant. Viví en Madrid durante diez años is the preterite, and ten years is a long time — but the living is finished, so it is one completed block. Estaba en casa cuando llamaste is the imperfect, and being at home lasted a few minutes — but it was the background to the phone call, so it is imperfect.

Preterite is what happened. Imperfect is what was happening.

the sentence that uses both

This is the shape you will be given in a translation or a reading text, and it is the clearest way to see the difference:

Leía un libro cuando sonó el teléfono. I was reading a book when the phone rang.

Two verbs, two different jobs. The reading was already going on — background, imperfect. The phone rang — one event that interrupted it, preterite.

Turn it round and the meaning changes with it. Leí un libro cuando sonó el teléfono says you read a whole book after the phone rang, which is a strange thing to have done.

what the two look like

Hablar, right across:

Two things worth noticing. The preterite lives on its accents — hablo is "I speak" and habló is "he spoke", and the only thing between them is a mark you can forget to write. And yo and él are identical in the imperfect, which is why the imperfect keeps its pronoun far more often than other tenses do.

The imperfect is the friendliest tense in Spanish to learn. There are three irregular verbs in the whole tense: ser (era), ir (iba) and ver (veía). Everything else takes -aba or -ía endings without argument. The preterite is the opposite — hacer, tener, estar, poder, poner, venir and decir all rebuild their stem, and they are among the commonest verbs there are.

the words that give it away

Some words nearly always sit with one tense, and spotting them is faster than reasoning it out from scratch.

Pointing at the preterite — a moment: ayer, anoche, el año pasado, de repente, una vez, dos veces, entonces.

Pointing at the imperfect — a habit or a backdrop: siempre, normalmente, todos los días, cada verano, a menudo, mientras.

Mientras is worth learning on its own. It means "while", and "while" is another way of saying "this was still going on", so what follows it is almost always imperfect: mientras hablaba, not mientras hablé.

descriptions are imperfect

Anything in a story that is scenery rather than plot goes in the imperfect. The weather, the time, someone's age, what a place looked like, how a person felt:

Hacía frío y eran las ocho. María tenía quince años y estaba nerviosa.

None of those is an event. They are the conditions the events happen inside. Then the plot arrives and switches tense: Entonces entró en la sala.

This is most of what a good narrative answer is doing. Set the scene in the imperfect, tell the story in the preterite, and the writing sounds Spanish rather than translated.

the verbs that change meaning

A short list, and it catches people out because both versions are correct Spanish — they just say different things.

The logic is the same one as the rest of the post. Knowing something is a state that goes on — imperfect. The moment you learned it is an event — preterite. And no quise is not "I did not want to"; it is "I refused", because the finished version of not wanting something is turning it down.

how to actually get it right

Understanding the rule and applying it under time pressure are different skills, and only the second one is being marked. What builds it is doing the choice over and over on real sentences until it stops being a decision.

That is what the preterite and imperfect drills on langrevise are for: both tenses across 414 verbs, taught before they are tested, and brought back on a schedule that gets longer each time you get one right. If you want the whole picture first, all nine tenses are here.