We use "Past Perfect" in the following situations:
1. To talk about an action that started in the past wich is before another action.
"I had read the news on internet before I watched it on the TV"
"she had known him before he became famous.
2. To talk about actions that happened in a specific time in the past.
"I had already taken a shower when my mom arrived at home".
(+) Affirmative: Subject + had + verb(participle form) + complement.
(-) Negative: Subject + had/hadn't + verb(participle form) + complement.
(?) Interrogative: Had + subject + verb(participle form) + complement.