The plus-que-parfait is a past tense that takes place before another action in the past tense, or an action which took place some time ago and in English is Had been or Had+the Past participle = had eaten, slept, skied, swum.

To form the PQP, you must follow the steps of the passe compose EXCEPT the helping verb is conjugated in the IMPARFAIT not the PRESENT TENSE. Otherwise everything else is the same. As an example Je vois ( Isee) in the present tense become J’ai vu in the Passe Compose. In lieu of ‘AI’ , avoir is conjugated in the Imparfait , or AVAIS.  Voir’s past participle is vu, which does not change in the PQP. Hence the PQP or je vois is = j’avais vu= i had seen.

2nd example: Elle part, becomes Elle est partie in the passé composé; Etre in the imparfait is ÉTais, ÉTais, ÉTait, ÉTions, ÉT iez, ÉTaient.  in the PQP: Elle Était (The ET are capitalized for effect) partie= she had left/departed.