Problem. English.

[xlade]

Hi teachers. I do not know when/where have had should be
used? Could you explain the purpose of "have had", like why it is used?

[Casiopea]

The first word, 'have' forms the perfect aspect: HAVE + VERB-ed/en. For example, have eaten, have seen, have walked, have studied, have had, and so on.

Simple Past
I had lunch.

Present Perfect
I have had lunch ~ I've had lunch. (present perfect)
She has had lunch ~ She's had lunch. (present perfect)

The paradigm for HAVE is as follows: I have, you have, she has, he has, it has, they have. Note, 'has' for 3rd person singular 'she', 'he', and 'it'.

Use the present perfect when the event is the main issue, and not the time the event happened. When it happened isn't important. For example,

Sam: Have you eaten?
Max: I've eaten. (Event: eat; When: not important)

Pat: Have you read this book?
Max: Yes, I have ~ Yes, I've read it. (Event: read; When: not important)

Max: Have you had lunch?
Pat: Yes, I have ~ Yes, I've had lunch. (Event: have lunch; When not important)