What is the difference between the past tenses (perfect, imperfect and pluperfect)?

The perfect tense (simple past) is used to describe a completed action. It is formed by an auxiliary verb (either être or avoir) and a past participle. 

The imperfect is used to describe an action which began in the past but is still on going - it is incomplete. It is also used as a descriptive tense to describe things seen/done etc in the past. It is formed by taking -ons off the 1st person plural form of the verb and adding the correct ending (-ais, -ais, -ait, -ions, -iez, -aient). 

The pluperfect can be translated as “had” and usually describes actions that happened before something else e.g. “I had done my homework when he arrived”. It’s formed by an auxiliary verb in the imperfect (either être or avoir) and a past participle