1. El pretérito, el imperfecto y el pluscuamperfecto

2. When do we use the preterit in Spanish?
The pretérito indicates:
I. A single event Fui a España el año pasado - I went to Spain last year.  
Visité Barcelona el sábado - I visited Barcelona on Saturday.
II. One or more events or actions that began and ended in the past Fui a España - I went to Spain.
Visité unos museos - I visited some museums.
III. An event that occurred, interrupting another action ...cuando me dijeron la verdad when they told me the truth. ...cuando mi hija nació - when my daughter was born.
IV. Changes in an existing physical or mental state at a precise moment or for a particular isolated cause Tuve miedo cuando vi el perro - I was scared when I saw the dog.

3. When do we use the imperfect in Spanish?
The imperfecto is used for:
A habitual or repeated action Iba a España cada año - I went (used to go) to Spain every year.
Visitaba mucho la Sagrada Familia - I often visited la Sagrada Familia.
II. An ongoing action with no specified completion Iba a España - I was going to Spain.
Visitaba unos museos - I was visiting museums.
III. Description/background information; set the scene of how things were or what was happening when there was an interruption Vivía en Costa Rica cuando... - I was living in Costa Rica when...
Estaba en mi cama cuando... - I was in bed when...
IV. General description of physical or mental states of being Tenía miedo de los perros - I was afraid of dogs.
V. Expression of the time of day or age in the past Eran las cinco de la mañana - It was five a.m.
Era sus cumpleaños; tenía doce años - It was his birthday; he was twelve.

4. Thus the imperfecto is normally used for descriptions of the past, while the pretérito narrates specific events.
In addition, the imperfecto often sets the stage for an event expressed with the pretérito.

5. Compare the following passages:
Imperfecto:
Cuando tenía dieciocho años, quería ser arquitecto. Me gustaba mucho la obra de Antoni Gaudí y esperaba comprender su genio. When I was eighteen, I wanted to be an architect. I really liked Antoni Gaudí's work and I hoped to understand his genius.
Pretérito:
Decidí estudiar en España e hice los formularios de inscripción, pero las universidades no me admitieron. Conocí a un pintor y comencé a estudiar con él. I decided to study in Spain and filled out the application forms, but the universities did not admit me. I met a painter and started studying with him.

6. Key words and phrases
The following list of key words and phrases may help you figure out whether to use imperfecto or pretérito.
Imperfecto: normalmente, de vez en cuando, antes (formerly), todos los días, los lunes, los martes, etc.
Pretérito: una vez, dos veces, tres/cuatro/etc. veces, muchas veces, ayer, un día, el lunes, de repente (suddenly, all of a sudden).

7. When do we use the imperfect in Spanish?
The pluperfect (aka past perfect) is used to indicate an action in the past that occurred before another action in the past. The latter can be either mentioned in the same sentence or implied.
Examples:
Ya había salido (cuando tú llamaste).
I had already left (when you called).
No habían comido (antes de hacer su tarea). 
They hadn't eaten (before doing their homework).
Fui al mercado por la mañana; ya había ido al banco.
I went to the store this morning; I had already gone to the bank.