When do you use the perfect tense as opposed to the imperfect tense?

In general, the imperfect tense is used to describe uncompleted actions whereas the perfect tense is applied when an action is complete. 

1. The imperfect explains what happened with no reference to specific time unlike in the perfect tense

e.g.

• J'étais à l'école

• Je suis arrivé tôt

2. The imperfect is used to describe repeated actions whereas the perfect tense expresses what happened a specific number of times 

• Je travaillais le lundi

• J'ai travaillé lundi 

3. Imperfect reflects an ongoing state or feeling but the past tense indicates a change of state

• J'aimais l'école 

• En ce moment, j'ai détesté l'école

4. The imperfect describes the background to something which is interrupted by the past tense

• J'étais à l'école quand il a commencé à pleuvoir.