The pretérito indefinido is used when an event happened (past tense) and took very little time to execute the action, or almost instantaneous, this implies that runs once. example: Suddenly a bomb exploded.-De repente una bomba explotó the above example gives the idea that the action happened only once. Because the bomb explodes only once. The pretérito imperfectot is used to express something that happened repeatedly in the past. Every Sunday I ate cake at breakfast. The idea is that the action to eat was repeated several times in the past.

Here I go: We use pretérito indefinido as you use Past Simple: I studied english - Yo estudié inglés I bought a car - Yo compré un coche And we use pretérito imperfecto as you use past continuous: While he was watching TV, his brother did his homework - Mientras él miraba la televisión, su hermano hizo los deberes. So, we use pretérito indefinido when something ended. I studied - Yo estudié / I bought - Yo compré / I read a book this morning - Yo leí un libro esta mañana / and we use pretérito imperfecto when we were doing something, something happened! Yo compré - it means that you bought something, that's all Yo compraba - I was buying. I hope it has helped you, XXOO

Actually, in spanish we don't have that tense. We use weather the "Pretérito perfecto simple" : Ayer me levanté a las once de la mañana or the " pretérito imperfecto" Hablabamos de la vida mientras reíamos. There are other past tenses but you'll find them like compound tenses.