Rules for past participle of has or have?

Answer 1

The verb is have

the past is had

the past participle is had

has is the third person singular form of have. Has is used when the subject of the sentence is he/she/it eg

She has a nice car.

Has is also used when the subject of the sentence is a singular noun eg

My sister has a nice car.

There are no rules for the past participles of irregular verbs ( have is an irregular verb because the past is not formed by adding -ed to the verb)

Answer 2

The past participle of "has" is "had", and the past participle of "have" is also "had". These forms are often used in perfect tenses, passive voice, or to show completion of an action in the past.