There is a song call “if I were a boy”, why not should be “if I was a boy” or “if I am a boy” which one is correct? And why?

See a translation

Whenever you are imagining a separate reality than what IS (now), use "were" (If I were...).

English is changing, and informally, you can also use "was" (If I was...)-DO NOT use "if *you* was".

The conditional "if" can refer to three situations:

the past, the (unknown) present/future,

...and the *known present*. IF you already know I'm not a boy, it makes no sense to say "If I am a boy" - because I'm not.

The only way this could be the case *now* is if we lived in a different (imaginary) reality.

We denote this imaginary reality with the past tense "were".

The only case where "if I am a boy" would be grammatically accurate, as if we are speaking but you do not yet know if I'm a boy or girl or not