Uses Of The Preterite

The main thing to keep in mind is that the preterite is used for one-time actions.  For instance, to say you walked to the park on saturday you would use the preterite:

Caminé por la calle el sabado pasado

Since the walking happened at a specific point in time, last saturday, you’d use the preterite.  However, actions that take place against a continuous backdrop of another action also take the preterite.  If you were walking to the park when you saw a dog, the verb “to walk” would take the imperfect tense while the verb “to see” would take the preterite.  That is because the focus of the sentence is on seeing the dog.  The walking is just background information.

Caminaba por la calle cuando ví un perro.

Uses Of The Imperfect

The imperfect tense usually signifies ongoing or background action.  Taking the example above, if you had used the imperfect tense instead of the preterite you would have been implying that you walked through the park regularly.  The sentence

Caminaba por la calle

indicates continuous action.  That is, it was not an isolated event.  Perhaps you walk down the street every morning on your way to work.  By contrast,

Caminé por la calle

implies this was a one-time action.  Perhaps you were forced to walk down the street just this once because your car had a flat tire.  Use of the imperfect implies repetition.

The imperfect tense is also used to describe background actions taking place in contrast to the main event.  In the example above,

Caminaba por la calle cuando ví un perro.

the verb “to walk” takes the imperfect tense, because the act of walking is merely a background action that happened to be going on when a discrete event, seeing the dog, occurred.  It’s what you were doing when you saw the dog.

This short lesson is a gross over-simplification, of course, but these simple rules will get you through 90% of your Spanish conversations.  For more on the preterite vs imperfect, check out my old Spanish verbs tutorial.