“Have” is the present simple form and “Had” is the past simple form of the verb “to have”.
I have a meeting today.
I had a meeting yesterday.
The verb “to have” means possession and is also used as an auxiliary verb in perfect(present and future) tenses in English.
Had is the past simple form of the verb “to have”.
Have is the present simple form of the verb “to have” for every subject except the third person singular(he/she/it).
We use the present simple for habits and things that are generally true. You can read more about the present simple here.
“Have had” is the present perfect form of the verb to have for every subject except the third person singular(he/she/it).
I have had
You have had
He has had
She has had
It has had
We have had
They have had
We use the present perfect tense for experiences and things that started in the past and continue into the present.
We use “had had” for the past perfect tense with the verb to “have”
I had had
You had had
He had had
She had had
It had had
We had had
They had had
When to use had in a sentence
We use had for past simple, past perfect and present perfect sentences.
I had a bad day yesterday.
I had been living there for one year before the tsunami.
I have had this watch for 5 years.
When to use have in a sentence
We use “have” for the present simple and perfect sentences.
I have 6 cats.
I have been here for a long time.
I will have been here for 6 years next month.