How can I know when to use the preterite or imperfect form of verbs?

Answer 1

Here are some sites with a good explanation of those two tenses.
http://www.drlemon.net/grammar/pretvsimp.html

http://www.studyspanish.com/lessons/pretimp4.htm

http://spanish.about.com/library/weekly/aa070302a.htm

Answer 2

Thank you for using the Jiskha Homework Help Forum. This was always one of my favorite things to teach!

Imagine, if you will, that you are going to paing a mural. You begin with the background (~ ~ ~ ~ ~ ). That is the imperfect, which in one word is "description." Note the English: was + ing, used to + infinitive, "ed" which overlaps with the Preterit. Now you paint the action. This is the Preterit. (/-/-/-/) Note the English: DID or "ed." The lines represent "ongoing event; we have no idea when it began nor if it ver ended = Imperfect vs. the lines that represent an action with a beginning and an ending = Preterit.

To illustrate: I used to be walking toward the door = salía = I was leaving the room. BUT I never go through the door way. This is the Imperfect. Then I walked to the door, through the door and around the corner = I DID leave the room. This is the Preterit. Happily my students always got the difference between the 2 past tenses. This makes the Spanish language so much "richer."

Sra