Both tenses are about the past and both are completed by the time you are telling the story, so "completed against ongoing" is no help mid-sentence. Ask instead whether you are running the camera or describing the set.
The camera and the set is a better test than completed and ongoing because it survives a whole paragraph. In "eran las ocho y hacía frío cuando salí", the first two verbs are the set — the time and the weather — and the third is the camera, the thing that happened. The last branch is the sentence shape that breaks beginners: when two things overlap, the one already underway takes the imperfect and the one that cut in takes the preterite, which is why "mientras cocinaba, sonó el teléfono" has one of each and never two of the same.
The words that give each one away
Spanish is unusually generous here: a handful of words almost always appear with one tense and almost never with the other. Learn the left column and half of the decisions are made before you reach the verb.
| The word in the sentence | Tense | A line you would hear in Mexico |
|---|---|---|
| ayer | preterite | Ayer comí tacos en el mercado |
| anoche | preterite | Anoche llegué a las once |
| el año pasado | preterite | El año pasado viajé a Oaxaca |
| de repente | preterite | De repente se fue la luz |
| dos veces, tres veces | preterite | La llamé tres veces |
| en 2019, a las ocho | preterite | Nos conocimos en 2019 |
| siempre, in the used-to sense | imperfect | Siempre íbamos a la misma taquería |
| todos los días, todos los domingos | imperfect | Todos los domingos hacía mole |
| antes | imperfect | Antes tomaba mucho café |
| mientras | imperfect for the one underway | Mientras cocinaba, sonó el teléfono |
| cuando era niño | imperfect | Cuando era niño jugaba fútbol |
| telling the time in the past | imperfect | Eran las ocho |
| somebody's age in the past | imperfect | Tenía diez años |
| the weather as scenery | imperfect | Hacía mucho frío esa noche |
The middle column is a shortcut and not a law: siempre can take the preterite when a speaker means "every single time, and then it stopped", and a sentence can carry ayer and still describe rather than report. But the shortcut is right often enough to be worth using out loud, and the four bottom rows have no exceptions at all — the clock, an age, and the weather-as-scenery are the imperfect every time, which is why "eran las ocho" is one of the highest-value four-word phrases a learner can own.
Twenty sentences, marked as you go
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Question 1 of 20 · 0 right so far
Ayer ____ tacos en el mercado.
Pick one to see the rule.
Each item gives enough of the sentence to decide, which matters more here than in the other two drills: por and para are settled by the words around the blank, but a past tense is settled by whether the speaker is reporting or describing, and a fragment with no ayer, no siempre and no cuando is a coin toss dressed as a question. Two of the four verbs that change meaning across the tenses are in the twenty — conocer and saber — because they are what a B1 learner is still getting wrong and what a table explains worst.
The four verbs that change meaning, not just tense
These are not exceptions to memorise and forget. They are eight things you can say instead of four.
Conocer. Conocí a Ana en 2019 is "I met Ana". Conocía a Ana is "I knew Ana", already, before the story started.
Saber. Lo supe ayer is "I found out yesterday". Lo sabía is "I knew it", all along.
Querer. Quise ayudarte is "I tried to help", and it carries the hint that it did not work. Quería ayudarte is "I wanted to help", which leaves the outcome open. In the negative, no quise is "I refused" and no quería is "I did not want to".
Poder. Pude abrirlo is "I managed to open it". Podía abrirlo is "I was able to", as a standing capacity. No pude is "I tried and failed"; no podía is "I could not".
The pattern behind all four: the preterite turns the verb into a MOMENT — the moment of meeting, of finding out, of trying, of managing — while the imperfect leaves it as a condition that was simply true.
The mistakes that mark a beginner
Using the preterite for the weather and the clock. Fue las ocho and hizo frío esa noche both sound wrong to a Mexican ear in a story. Eran, hacía.
Putting both halves of an interruption in the same tense. Mientras cocinaba, sonaba el teléfono says the phone rang for the entire time you cooked. One of each.
Translating "was -ing" as the imperfect every time. It is usually right, but "estuve leyendo toda la tarde" is a real and common sentence: the reading was bounded by the afternoon, so it takes the preterite.
Avoiding the imperfect because the endings feel easier. The imperfect has three irregular verbs in the entire language — ir, ser and ver. The preterite has dozens. The tense that feels harder is the one with almost no exceptions.
What to say when you are stuck
Ask what the listener needs. If they need to know what happened next, use the preterite. If they need to picture the scene, use the imperfect.
If you still cannot tell, use the preterite and keep going. A story told entirely in the preterite sounds flat and is perfectly clear; a story told entirely in the imperfect sounds like it never actually got anywhere.
Where the lessons on this site fit
The sibling choices, written to this page's shape, are ser vs estar and por vs para; the subjunctive is the one that arrives after these.
Past tenses come at you constantly in the Speak dialogues, because telling somebody what you did yesterday is most of what people ask each other. The B1 Chapter Books shelf is where the two tenses start alternating paragraph by paragraph, which is the reading that teaches this better than any rule.
Key takeaways
- The test is the camera against the set. What happened is the preterite; what things were like is the imperfect.
- Two things at once always takes one of each: the one already underway is imperfect, the one that cut in is preterite.
- The clock, an age and the weather-as-scenery are the imperfect, with no exceptions. Eran las ocho, tenía diez años, hacía frío.
- Ayer, anoche, de repente and a counted number of times pull the preterite; siempre, antes, todos los días and cuando era niño pull the imperfect.
- Four verbs change meaning: conocí is met and conocía is knew, supe is found out and sabía is knew.
- The imperfect has three irregular verbs in the whole language. The tense that looks harder has fewer traps.
- When stuck, use the preterite and keep talking. Flat and clear beats stalled.