Using The Past Perfect:
Like all perfect tenses the past perfect is used with yet, for, since, so far, and already . It also expresses an action prior to nother action and the notion of time of the action is not important.

Use: Remember that there must be an action already completed in the past to use the past perfect.
Past Perfect ----- Action in the past ---- Present
l__________________________l__________________l
the report competed -- boss arrived -------- now

Examples:
He had already completed the report when his boss arrived.
Had he not finished the project before the meeting?
The team hadn't met before the trip.

Structure:
Subject + had + past participle.
Interrogative
Had+subject+past participle
Negative
Subject + had not/had’t+past particle.