Both, Spanish preterite (or preterit) and imperfect tenses are used to express past actions, but in English we have just one past tense. So, the English past form "I ate" could either be translated as "yo comí" (preterit) or "yo comía" (imperfect) depending on what we want to express (notice that they aren't interchangeable).
But how do we know when to use either the imperfect or the preterit tenses? In general terms, the preterit is used to express an action (in the past) that is completed or seen as completed, that has concrete and clear beginning and end. So, we use the other tense, the imperfect, as its name shows, when referring to actions that haven't got a clear or concise end.
We could also understand the preterit as a tense that give us specific information on when the action took place. On the other hand, the imperfect informs us generally about the end of the action.
Ok, let's go further; these are some 'tips' to help us with the use of both Spanish past tenses:
Use the preterit:
- If you want to talk about something that just happened once:
Mi hijo nació el miércoles pasado (My son was born last Wednesday)
Ayer llamé a mi hermano (Yesterday I phoned my brother).
- to tell when something began or ended.
La película comenzó a las siete y terminó a las ocho y media.
(The film started at seven o'clock and ended at half past eight.)
- There are some words and expressions, indicating concrete time frames, so they are used with the preterit: ayer (yesterday) antesdeayer (the day before yesterday), en ese momento (at that moment), entonces (then) la semana pasada (last week), el mes pasado (last month), esta tarde (this evening) esta mañana (this morning).
Use the imperfect:
- if you want to talk about repeated actions (or actions made habitually):
Ella comía arroz todos los días. (She aterice everyday)
Mi tía siempre llegaba tarde (My aunt was always late)
- if you want to talk about time or age in the past:
Él tenía 20 años. (He was twenty years old)
Era medianoche. (It was midnight)
- There are also some words and expressions that are used with the imperfect: cada día (every day), a veces (sometimes), de vez en cuando (from time to time), generalmente (usually). Mucho (a lot), siempre (always), nunca (never). These are expression that do not point to a specific point of time, that's why they are used with the imperfect tense.
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¡Que tenga un buen día!