What is the difference between the usage of the imperfect an the present perfect ?

Well, it is important to understand that the imperfect is a tense used to describe a habitual or continuous action in the past. It can be used to describe the environment of a situation or simply an action which occurred over a prolonged amount of time repetitively eg: I used to always go to the cinema. The present perfect however is used to describe an action which is complete, you can usually tell when an action is said to be complete if time is involved eg: I went to the shop on Friday.