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. As was mentioned in §28, 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

See §22 to review the difference between conocer and saber

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.

The original website of this content + some exercises here.