Hola. You use "pretérico perfecto simple" when you contemplate the whole action and you know it finished: Pedro vivió en Buenos Aires -> so... he no longer lives in Buenos Aires... he moved to another place... or he died. You use "pretérito imperfecto" when you focus that moment when the action took place... it is like if focusing so much on it allows you to develop the action, to describe it, etc... but you are focusing so much that you can not tell if it finished or not: Pedro vivía en Buenos Aires -> Can´t tell if he is still living there or not.. or if he died or not... You use pretérito imperfecto for some other special situations: a common one is to describe habits from the past: Mi amigo tomaba cerveza todos los sábados. The other ones.. let´s skip them XD But it mainly appears in this contexts, if you read some text that tells a story in the past, you will notice it combines this two different past a lot, "preterito perfecto simple" develops the main plot and "pretérico imperfecto" describes the stage, actions that took place simultaneously with "preterito perfecte simple", habits, etc. For example: Cuando yo era chico tenía un perro (I describe something..not sure if I still have this dog or not) , se llamaba Pipo. El salía a pasear todos los días (habit). Un día un camión lo atropelló (the plot starts...we know he actually got hit by a truck), por suerte Pipo que era muy fuerte (I describe) salió ileso (the plot continues...we know he did not get hurt). Hope it helps :)

You must use the preterito perfecto simple when the action has already finished. E.g: - Cogí el libro - Hablé con él You must use the preterito imperfecto when the action hasn't finished yet or it's in progress - Estudiaba en mi habitación - Estaba leyendo cuando llegaste It's the basic rule, but it's not all. Sorry for my english