The preterite is a "perfect" tense because it reports events viewed as completed within a finite time frame (a change, a definite beginning or end at a specific point). An "imperfect" tense conveys duration, progression, incompleteness (a state, something experienced for some time):

Los indígenas no aceptaban a los europeos.
The natives would not accept Europeans (for some time).
Algunos nunca los aceptaron.
Some (definitely) never did.

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

podercould, had the ability tomanaged to (or failed to if negative)
quererwanted, had the intention totried to (or refused if negative)
conocerpreviously knew for some timemet, got to know for the first time
saberhad knowledge for some timelearned, found out at a specific point

Review the difference between conocer and saber (§22)

Pulse la forma verbal que mejor completa la narración. Click on the verb form that best completes the narration.
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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.

(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.