The past perfect is very similar to the present perfect: the auxiliary verb "have" goes one step back in time and becomes had.
We generally use the past perfect when we have two actions in the past and we want to show that one action completed before the other one started. The past perfect can also serve as an equivalent of the present perfect for the past.
I had come home before it started to rain.
The concert had already finished when we arrived at the concert hall.
Antonio started a new unit of grammar after he had completed the previous one.
The milk was fresh. I had checked the date on the bottle. Compare: The milk is fresh. I've checked the date on the bottle.
Andrew couldn't believe he had won. Compare: Andrew can't believe he has won.
How to form the past perfect
had + past participle
|Pronoun||Auxiliary verb "had"||Past participle of the main verb|
You can read more on how to form and how to use the past perfect in the related topics.