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