How do we form the pluperfect for regular verbs?

The pluperfect refers to an action at a time earlier than a time in the past already referred to. For example: "j'avais mangé avant de partir".

So to form the pluperfect tense we use the imperfect form of the verb 'avoir', which means 'to have', and the past participle of the other verb. The imperfect forms of avoir are: j'avais, tu avais, il/elle/on avait, nous avions, vous aviez, ils/elles avaient. Then to one of these verbs we add the past participle for example mangé which is the past participle of the verb 'manger', which means 'to eat'. 

So we can then form a phrase like: "j'avais mangé" which means I had eaten.