How do you differentiate between the perfect tense and the imperfect tense?

The perfect tense is used for completed actions in the past, or for actions in the past that have an impact on the present. It can translate to the English perfect tense, but also the English preterite. For example:

I have already gone to the cinema this week  - je suis déjà allé(e) au ciné cette semaine

Last week, I went to a football match - la semaine dernière, je suis allé(e) a un match de foot 

In both cases, the perfect refers to a 'perfectly' completed action, which can be defined in time. The imperfect tense, on the other hand, refers to habits or things you did regularly in the past; 'I used to do', 'I would do', etc. It is also used for descriptions, and will often be found in narrative sections of books. For example:

Dans la forêt, il faisait beau: les oiseaux chantaient, un vent léger soufflait dans les pins et les enfants écoutaient l'eau de la rivière qui coulait près de leurs pieds. Tout d'un coup, un ours énorme est sorti de l'ombre.

In the forest, it was a nice day: the birds sang, a light wind blew through the pine trees and the children listened to the river water that ran near to their feet. All of a sudden, an enormous bear came out of the shadows. 

The actions are not completed in the past. The birds continue to sing, the wind continues blowing. Only the sudden apparition of the bear is in the perfect tense: he appears, then stops appearing because he is already in front of the children.