Why do we use “I have a book” instead of “My have a book”?

“I have a book” just states that you have a book.

If you want to say the book is YOURS, you say,

“I have MY book.”

“This is my book.”

“This book is mine.”

…or some variation of the above.

I for the first person who speaks

I have a book

you have a book

We have a book

my means this thing belong to me.

my book not your book

the book belongs to me.

my what?

my flower, my teacher, my wife, my husband,…

Hence “my have a book” has no meaning in English

It must be “I have a book”

I is correct. My is a possessive pronoun that you wouldn’t use first in a sentence.

Simply put, because “I” is correct and “my” is wrong. “my” is first person singular POSSESSIVE, “I” is first person singular SUBJECTIVE which is what you want.

‘I’ is the subject in this sentence. ‘I’ is therefore a subjective pronoun, designed to replace a person (as is he, she, we, you, they). ‘My’ indicates possession and is called a possessive pronoun (as is your, his, her, our, their).