When do you use the Preterito perfecto and when the Preterito Indefinido?

You should think of the preterito perfecto as the present perfect in English and the preterito indefinido as the past simple. Indeed, you use the preterito perfecto (present perfect) when an action which has started in the past is still going on or has a consequence on the present. Ex:-You are all wet! Where have you been?- -I have been outside and it is raining--¡Estás todo mojado! ¿Dónde has sido?--He sido afuera y está lloviendo- In this example the boy was outside in the rain and when he went back into the house, although he was no longer in the rain, he was still wet because of it and that is why we used the preterito perfecto (present perfect).There are also some words to look out for that can help us decide whether to use the preterito perfecto. Some of these are: nunca, a veces, siempre, casi nunca, este/a mes/semana,... and many more!On the contrary, the preterito indefinido is used when an action starts and ends in the past and has no influence on the present. Words that can help us are: ayer, anteayer, anoche, ese, aquel, hace, ...