The difference between the past imperfect and past perfect.

Question

Puede explicar con mas detalle cuando se usa el preterito imperfecto y cuando se usa el preterito perfecto, por favor?

Answer

Hola William
First of all, apologies for mentioning in the introduction to this exercise that you would be practising El Imperfecto - that was a typo. The focus of this exercise is El Perfecto, not El Imperfecto.
Secondly, we don't have any lesson explaining the contrast between the imperfect and the perfect, as this is not normally an issue. But we have independent lessons that deal with the different uses of each tense. 
Here are some of them:
El Imperfecto
El Perfecto
Each link will take you to information about the tense and a list of lessons will appear at the bottom. If you want to learn about the usage of these tenses, go to the lessons with titles "Using... " (not the conjugation ones)
I hope it helps
Saludos
Inma