How do you know when to use the imperfect or preterite
Carmen (ir) de compras porque (buscar) ropa nueva para el colegio.
Carmen went to the mall because she went to look for new clothes for school.
I don't need help congugating the word for me, I know how to do that part. But, could someone explain the reasons why the words should be imperfect or preterite.
should (ir) be in the preterite form or imperfect form and should (buscar) be in the preterite or imperfect form?
The perfect tense or Preterite is used to indicate an action that has been completed at some point in the past
for example; (él) Corrió por la calle = He ran through the street
or Compré un reloj de pulsera para mi sobrino = I bought a watch for my nephew.
It is used to indicate an action that has happened only once
The imperfect - by contrast - is used to indicate an action that a person was doing or used to do: the regular ar verbs will end in aba and the Ir and er verbs in ía
(Yo) Trabajaba con los niños en una escuela = I used to work with children in a school
Estaba leyendo cuando mi hermano entró la sala (de estar) = I was reading when my brother entered the living room
The last sentence illustrates quite well the use of both verb tenses in the same sentence
This is just intended as a basic guide please also look at the verbs under the grammer section Pajaro has given you a link on her post
I hope that this helps
Here is another resource, on this Web site, for explanations on the use of the "preterite" and the "imperfect":
All you need to do is click on the topics preterite and imperfect after going to the link.
From my (limited) understanding, the preterite is used if the action occurred at a specific time and the imperfect is used if you don't really know when it happened. See this for another explanation: http://www.studyspanish.com/lessons/pretimp1.htm and this: http://spanish.about.com/od/verbtenses/a/two_past_tenses.htm
Example: The Spanish verb "venir" gives us the means to express "to come." One of the neat things about "venir" is that this verb is very similar to another verb you have already learned. Look at the forms below and see if you can guess which verb. That's it! Tener!
ir- to go
ir- to go
venir -to come
él, ella, usted
ellos, ellas, ustedes