Hi Antonia I will try to explain you as well as I can.

both are use to describe past sentences, the only different is about how long the action took place in the past.

Preterito imperfecto describe an action that it was repeated many time in the past or it took longer. Also this tense doesn't describe if the action finished or not. Preterito indefinido is like the simple past in english, just a action that happened only one in the past . Examples

Preterito imperfecto:

Yo caminaba: That mean I was walking and took me a period of time and I trying to making clear that probably I kept walking or I stopped,

Preterito indefinido

Yo caminé: That mean I walked, nothing else. just a dot in line time.

Hello Antonia

I'm trying to explain it. My English is not too good, sorry.

We use the preterito imperfecto for repeated actions in the past. For example:

I cooked the dinner EVERYDAY.

Yo cocinaba la cena todos los días.

He always went to school by bus.

El siempre iba al colegio en autobús.

We use it to express continuity in the past.

He loved football.

Le encantaba el fútbol.

They were  busy all the time.

Ellos estaban ocupados todo el tiempo.

We use the "pretérito indefinido" for completed actions in the past.

I cooked the dinner YESTERDAY. (We don't know if I cook everyday or not but the action is finished so we use pretérito indefinido)

Yo cociné la cena ayer.

Last week he went to school by bus.

La semana pasada (él) fue al colegio en autobús.

We use it to express a new action in the past in the middle of another action.

I was married (1) when I met her (2)

(1) it's an action with continuity in the past so it is "pretérito imperfecto".

(2) it's a new action in the past in the middle of another one, so it's "pretérito indefinido".

Yo estaba casado cuando la conocí.

My son was running and he suddenly fell.

Mi hijo estaba durmiendo (3) y de repente se cayó

(3) it works as past continuous in English.

Espero haberte ayudado :)