“Happier” is correct in English because the adjective “happy” ends in “y”.
The rule in English is that when you have an adjective that ends in “y”, you need to drop the “y” and add “ier” to make the comparative form.
I am happier than you!
Is more happier grammatically correct?
No, “more happier” is not grammatically correct as there is no need to use the word “more”. “Happier” is the correct form of the word.
Is “most happy” correct?
No, “most happy” is incorrect. The correct superlative form of the word is”happiest”.
This is the happiest day of my life.