Should I Use the Preterite or Imperfect Past Tense?
Spanish has two past tenses: preterite and imperfect. Most verbs can be put into either tense, depending upon the meaning.
I've found plenty of Spanish learners struggle with the difference between the two. I'll explain in the easiest way that I can.
Generally, imperfect is used for actions in the past that are not completed. Often, imperfect tense means that the past action didn't have an obvious beginning or end.
The imperfect tells us in general when an action took place.
Imperfect also is used for setting a scene, or describing something in the past. 'Ser' is almost exclusively used in imperfect.
--I was walking. (No real beginning or end to this action)
Estaban hablando cuando la película comenzó.
--They were talking when the movie started. (Setting up for an interruption)
Cuando era niño...
--When I was a child... (description)
Most often, preterite is used for actions in the past that are completed. Use of the preterite tense implies that the past action had an obvious beginning and end.
The preterite tells us specifically when an action took place.
Occasionally, preterite follows imperfect because it is an interrupting action. I was doing ____ when suddenly I ___!
Él estaba caminado cuando se cayó.
--He was walking when he fell. (an interrupting action)
Mi primo habló dos horas.
--My cousin talked for two hours (a completed action)
So basically, if the action is completed in the past use preterite. If the action is an ongoing action or a description, use imperfect.