1.
Had is the past tense and past participle of have .
2.
Had is sometimes used instead of ‘if’ to begin a clause which refers to a situation that might have happened but did not. For example, the clause ‘~ he been elected’ means the same as ‘if he ~ been elected’.
Had he succeeded, he would have acquired a monopoly...
Had I known what the problem was, we could have addressed it.
AUX: AUX n -ed, AUX n -ed
3.
If you have been ~, someone has tricked you, for example by selling you something at too high a price. (INFORMAL)
If your customer thinks he’s been ~, you have to make him happy.
PHRASE: be inflects
4.
If you say that someone has ~ it, you mean they are in very serious trouble or have no hope of succeeding. (INFORMAL)
Unless she loses some weight, she’s ~ it...
PHRASE: AUX inflects
5.
If you say that you have ~ it, you mean that you are very tired of something or very annoyed about it, and do not want to continue doing it or it to continue happening. (INFORMAL)
I’ve ~ it. Let’s call it a day...
PHRASE: AUX inflects