Past Perfect (had + Past Participle) expresses an action which happened before another past action or before a stated time in the past.
By the time he showed up for the meeting, it had already finished.
Add "not" to "had" to make a negative sentence.
I hadn't finished my homework when you came.
"Had" is used to ask questions and give answers.
Had you met him before he was introduced to you at the party? - No, I hadn't.