Spanish uses two indicative tenses to refer to the past: the preterite and the imperfect. The two tenses are not interchangeable without altering the meaning of the sentences, and each one possesses its own well-defined logic. At their most basic level, the preterite tells what happened, and the imperfect describes how things were.
The preterite tense is often
referred to as the narrative past. It reports discrete,
completed past actions, which may have occurred once or many times
and may have had infinitely short or unimaginably long duration.
The preterite is often, but not always, associated with key words
such as ayer, anteayer, el año pasado, de repente,
comenzar, empezar, and iniciar, among others.
Where the preterite tense is often referred to as the narrative
past, the imperfect tense is often
called the descriptive past. Its focus is on less discrete
units of activity. Rather, the imperfect reports past repeated
actions or things that "used to" happen, and it describes
activities and conditions amid the passage of time. The imperfect
is often, but not always, associated with key words such as
siempre, mucho, a menudo, frequentemente, a veces, and
soler.
The two past tenses often occur within the same sentence. When this happens something that is unfolding in time is punctuated by the occurence of some other action. That is, a contextual situation is interrupted by a discrete action:
Several verbs in Spanish when expressed in one past tense change
their meaning noticeably when expressed in the other. It should be
mentioned, however, that the careful user will notice that these
"special meanings" are not inconsistent with the general guidelines
on this and other pages for the use of the preterite and the imperfect. They are noted here simply because
they are commonly used verbs and because they provide convenient
pairs for comparison purposes.