Imperfect and Preterite Contrasted 

A. The preterite is a “perfect” tense because it reports events viewed as completed within a finite time frame. An “imperfect” tense conveys duration, progression, incompleteness:

IMPERFECT (duration) PERFECT (PRETERITE: completion)
Los indígenas no aceptaban a los europeos.
The natives would not accept Europeans (for some time).
Algunos nunca los aceptaron.
Some (definitely) never did.
El sábado nevaba y hacía frío.
It was snowing on Saturday, and it was cold. (for some time)
El sábado nevó todo el día.
It snowed all day on Saturday. (time frame)
El año pasado asistía mucho a conciertos.
Last year I used to attend concerts a lot (often).
El año pasado asistí a muchos conciertos.
Last year I attended many concerts.

B. For narration, each tense has a distinct function:

IMPERFECT (duration) PRETERITE (completion)
gives background information,
describes what was happening
reports completed actions,
tells what happened
It was raining…
Llovía (o estaba lloviendo)
when I woke up,
cuando me desperté.
describes a state or condition reports a change in condition
He was fine before the winter, …
Estaba bien antes del invierno, 
but in January he got sick because of the cold.
pero en enero se enfermó por el frío.

C. A few verbs have slightly different meanings in the preterite. All of them, however, follow the general idea of duration for the imperfect and of completeness for the preterite:

Imperfect Preterite
poder could, had the ability to managed to (or failed to if negative)
querer wanted, had the intention to tried to (or refused if negative)
conocer previously knew for some time met, got to know for the first time
saber had knowledge for some time learned, found out at a specific point

Los conocía antes de viajar.
I knew them before traveling.
Los conocí al viajar.
I met them while traveling
¿Qué sabía Bush del ataque?
What did Bush know of the attack?
¿Cuándo supo Bush del ataque?
When did Bush find out about the attack?
No sabía que eras chilena.
I did not know you were Chilean.
Ayer supe que eras de Chile.
Yesterday I learned you were from Chile.
De niño podía jugar todo el día.
As a child, I could play all day.
También pude hacer muchos amigos.
I also had the opportunity to make many friends.
Como no podíamos salir, …
Since we couldn’t go out, …
no pudimos ver los fuegos artificiales.
we were unable to see the fireworks.
Queríamos ir de compras, …
We wanted to go shopping, …
pero mi madre no quiso darnos dinero.
but my mother refused to give us some money.

Practice

Use the translation to choose the correct form.

(1) I didn’t know anything about Colombia before, but last December (2) I traveled to Bogotá and (3) learned a lot about the country. During my visit, (4) I became familiar with several shopping malls (for the first time), dancing clubs, universities, and huge parks. I also (5) managed to see Fernando Botero’s originial works –the famous artist with the voluminous shapes–. On our last day, a turist guide (6) told me that (7) I could not leave without seeing the Gold Museum, and immediately (8) I decided to visit it. Fortunately, that same afternoon (9) we managed to go to the museum. (10) There were impressing gold pieces elaborated by the Muisca culture, the indigenous group from that region. We also (11) learned that the Muiscas always (12) refused to reveal the secret of El Dorado –the legendary golden city– to the Spaniards, who (13) failed to find it at all. Paradoxically, Colombians (14) gave the capital’s international airport the name of “Eldorado”. Thanks to this trip, I (15) learned that Bogotá is a very modern city with a rich history.

Antes yo no (1. sabía/supe) nada sobre Colombia, pero el diciembre pasado (2. viajaba/viajé) a Bogotá y (3. aprendía/aprendí) mucho sobre el país. Durante mi visita, (4. conocía/conocí) varios centros comerciales, discotecas, universidades y parques inmensos. También (5. podía/pude) ver las obras originales de Fernando Botero, el famoso pintor de figuras voluminosas. El último día, un guía turístico me (6. decía/dijo)que yo no (7. podía/pude) irme sin ver el Museo del Oro, e inmediatamente (8. decidía/decidí) visitarlo. Afortunadamente, esa misma tarde (9. podíamos/pudimos) ir al museo. Allí (10. había/hubo) impresionantes piezas de oro elaboradas por la cultura Muisca, los indígenas de esa región. También (11. sabíamos/supimos)que los muiscas nunca (12. querían/quisieron) revelar el secreto de El Dorado –la legendaria ciudad de oro– a los españoles, quienes no (13. podían/pudieron) encontrarlo jamás. Paradójicamente, los colombianos (14. daban/dieron) el nombre de “Eldorado” al aeropuerto internacional de la capital. Gracias a este viaje, (15. sabía/supe) que Bogotá es una ciudad muy moderna con una rica historia.

 

(source)