Preterite vs. Imperfecto

The preterite is used for past actions that are seen as completed. The imperfect tense is used to refer to actions in the past that occurred repeatedly or indefinitely.

My profe describes the preterite as a "snap", it happened, it's over, terminado. The imperfecto, she says, is like a wave, it comes it goes. You can't put your finger on it. Think of the imperfecto as descriptive.

Comí la manzana que estaba sentado en la mesa.

I ate the apple that was sitting on the table. Snap, the apple is gone. Notice that I added a past participle, "was sitting", estaba sentado. Here the past participle is used like an adjective.

Compare:

Yo comía manzanas cada dia. Ayer, me comí la última manzana.

I ate apples every day. Yesterday, I ate the last apple.


Eschuco -     I am listening
Escuchaba - I was listening
Escuché -     I listened
Escucha -     Listen!

Escucha la canción Chau

For more, check out the page on stem changing verbs that list both present and preterite tenses for common stem changing verbs. 

Preterite and Imperfect Endings - ar, er, ir

ar é aste ó amos asteis aron	ar aba abas aba ábamos abais aban
er, ir é iste ió imos isteis ieron	er, ir ía ías ía íamos íais ían