Using the imperfect vs the preterite in Spanish (general use)

To talk about actions that took place in the past we can use both El Pretérito Imperfecto and El Pretérito Indefinido. 

Which one you need to use depends on whether or not you consider an action in the past to have a beginning and an end. If you do, you should use El Pretérito Indefinido, if you don't then you should use El Pretérito Imperfecto. This can be tricky for English speakers to understand because both tenses are usually translated the same in English.

Let's have a look at these examples:

In the first example where El Pretérito Imperfecto is used (abría) we can visualize a door in the process of being opened however in the second example where El Pretérito Indefinido is used, we can see a completely open door.

So the main difference between using these two tenses is:

Have a look at other examples in both tenses:

Trabajaba = we see Susana working.

Trabajó = we see Susana when she has finished her work.

Corrían = we see the boys running.

Corrieron = we see the empty corridors after the boys ran.

Comía = we see Antonio eating his sandwich.

Comió = we imagine Antonio throwing his sandwich wrapper away.

See also Using the imperfect vs the preterite in Spanish (time markers).