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 you (or the speaker) consider the beginning and end of the action, or if you consider the action to be completed. 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:
La profesora abría la puerta.
The teacher opened the door.
La profesora abrió la puerta.
The teacher opened the door.
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:
El Pretérito Imperfecto for actions in the past with no clear beginning and end.
El Pretérito Indefinido for actions in the past with a clear beginning and end.
Have a look at other examples in both tenses:
Susana trabajaba muy duro.
Susana worked very hard.
Susana trabajó muy duro.
Susana worked very hard.
Trabajaba = we can see Susana working.
Trabajó = we can see Susana when she finished her work.
Los chicos corrían por los pasillos haciendo ruido.
The boys ran along the corridors making noise.
Los chicos corrieron por los pasillos haciendo ruido.
The boys ran along the corridors making noise.
Corrían = we can see the boys running.
Corrieron = we see the empty corridors after the boys ran.
Antonio comía un bocadillo a mediodía.
Antonio had a sandwich at midday.
Antonio comió un bocadillo a mediodía.
Antonio had a sandwich at midday.
Comía = we can see Antonio eating his sandwich.
Comió = we can see Antonio throwing his sandwich wrapper away.
See also Using El Imperfecto vs El indefinido (time markers).
Los chicos corrieron por los pasillos haciendo ruido.
The boys ran along the corridors making noise.
Antonio comía un bocadillo a mediodía.
Antonio had a sandwich at midday.
Los chicos corrían por los pasillos haciendo ruido.
The boys ran along the corridors making noise.
Susana trabajó muy duro.
Susana worked very hard.
La profesora abrió la puerta.
The teacher opened the door.
Susana trabajaba muy duro.
Susana worked very hard.
La profesora abría la puerta.
The teacher opened the door.
Antonio comió un bocadillo a mediodía.
Antonio had a sandwich at midday.
