Two Aspects of the Simple Past Tense in Spanish

Reflects both the beginning and end without reference to either one, but simply as a concluded past event.

Pasé diez años en España.
I spent ten years in Spain.

As can be seen from the above examples, any reference to a past beginning of an action/event/state takes the preterite as do references to a past ending or to an action/event/state which is completed and is considered from the point of view of its completion.

"Imperfect," on the other hand, is, by definition, incomplete or continuing. Thus, when we conceptualize a past action/event/state as it was unfolding without reference to when it began or when it ended, or, when we refer to a past action which was repeated (again, without reference to a specific period of time or to an end point,) Spanish uses the imperfect. Consider the following sentence:

Cuando yo tenía diez años, jugaba con mi perro que se llamaba Sydney.
When I was ten, I played with my dog whose name was Sydney.

I am now 82 years old and Sydney has been dead for 70 years and so both my being ten years old and my dog Sydney are past completed sentences. However, my focus is neither on the beginning of my tenth year, nor on the end of my tenth year, nor on the completed past experience of being ten and playing with Sydney, nor on Sydney's death nor on his being dead. Thus, the verbs are in the imperfect.

The imperfect is also used when we speak of past repeated or habitual actions/events/states without reference to a conclusion. For example:

No se llevaban muy bien.
They did not get along well.

However, if you indicate a specific time frame in which the action/state/event took place thereby indicating that it is clearly a concluded past, the verb is in the preterite. An example would be:

Después de la muerte de su hijo, no se llevaron bien por un año.
After the death of their child, they did not get along well for a year.

As you no doubt know from previous study of Spanish and from our temporal map, it is not uncommon to find both the preterite and the imperfect in the same sentence. If you have a past action/state/event that was ongoing and therefore, makes no reference to when it began or to when it ended (Ex.: "I was studying.") and the action of another verb intercepts the action of the verb which reports the ongoing action, then the second verb will be in the preterite while the first verb will be in the imperfect.

Yo estudiaba cuando María me llamó.
I was studying when María called me.

If, on the other hand, María called me each and every time that I was studying, then the action of both verbs is habitual and both will be in the imperfect.

Try practicing as much as you can doing exercises on this page so that you can begin to familiarize yourself and become comfortable with the use of preterite and imperfect aspects of the simple past tense. This is very important because in the world of discourse, both speaker and listener must have the same understanding of the nature of what the language expresses and as you will begin to realize, if you use the wrong verb form, you communicate the wrong message.