El uso del Pretérito 

Whether you should use the preterite or the imperfect depends in a large extend on the meaning of the verb that you are using. 

There are basically two kinds or verbs:

action and punctual verbs like  golpear (to hit), saltar (to jump), comprar (to buy), vestirse (to get dressed) etc., verbs used to say that someone is doing something and that what you are doing implies somehow movement or change, and that the action is relatively short

verbs of state like ser (to be), estar (to be), querer (to want), tener (to have) etc., verbs used to describe longer lasting conditions or states.

Again, this is only a rule of thumb because there is no clear cut between action verbs and verbs of state, and depending upon the context the same verb can be interpreted a describing a state or an action, but as ageneral rule we can say

1. action verbs are essentially used in preterite :

Me levanté a las 7, me duché, desayuné y fui a clase. (I got up at 7, took a shower, ate breakfast and went to class.)
The use of the imperfect in this case would imply that you did this repeatedly or habitually, over an unspecified period of time:
Me levantaba a las 7, me duchaba, desayunaba e iba a clase. (I gused to get up at 7, take a shower, eat breakfast and go to class.)
But the important thing here is the unspecified time frame. Repetition can also be seen as a completed series. In this case again you use the preterite:
En 1999 todos los días me levanté a las 7, me duché, desayuné y fui a clase. (In 1999, every day I got up at 7, took a shower, ate breakfast and went to class.)

and therefore:

2. if the action is dated, this means if there is a time indicator or a time frame specifying the beginning, the end or beginning and end of the action, use the preterite. The same goes for verbs describing states: If you limit the time frame for the state, you use the preterite. (By doing this you imply a change of state and therefore an action.)
Ayér jugué al dominó con mi hermano. (Yesterday I played domino with my brother.)

El 31 de diciembre de 1999 estuve en Nueva York. (On december 31 of 1999 I was in New York.)

Me alegré al oir la noticia. (I became happy when I heard the news.)

Some time frame limiters are: ayer (yesterday), anteayer ( the day before yesterday), anoche (last night), el año pasado (last year), el mes pasado (last week), el lunes pasado (last Monday), hace tres días (three days ago), etc.
 

ojo2.gif - 639 BytesThere are some verbs of state that actually change meaning when used in the preterite:
      • Conocer: to know / to meet ( as in beeing introduced to) or get to know
        • Yo la conocía desde niña. (I knew her since she was a child) but:  La conocí ayer en clase. (I met her yesterday in class.)
      • Saber: to know / to learn or find out or to realize
        • Yo ya sabía la noticia.(I already knew the news.) but: Yo supe la noticia ayer. (I found out about the news yesterday.)
      • Poder: to be able, can / to succeed or manage
        • Podíamos salir a las tres. (We could leave at three.) but: Pudimos salir a las tres. (We managed to leave at three.)
      • No poder: not to be able / to fail
        • No podíamos abrir la ventana. (We were not able to open the window.) but: No pudimos abrir la ventana.(We failed to open the window.)
      • Querer: to want / to attempt, try
        • Juan quería ir al cine. (Juan wanted to go to the movies.) but: Juan quiso ir al cine pero no tuvo tiempo.(Juan tried to go to the movies but he didn't have time.)
      • No querer: not to want / to refuse to
        • Juan no quería ir al cine. (Juan didn't want to go to the movies.)but: Juan no quiso ir al cine.(Juan refused to go to the movies.)
      • Tener que: Obligation, not necessary fulfilled / fulfilled obligation
        • Juan tenía que trabajar pero fue al cine. (Juan had/ was supposed to work, but he went to the movies.) but: Juan tuvo que trabajar anoche. (Juan had to work last night [and did].)