Learn Arabic (Modern Standard): هل هذا الطابع لك؟ نعم، هو لي. - Is this stamp for you? Yes, it is for me.

Both start with the preposition لـ meaning "for" or "to". In لك (for you), we attach the pronoun ـك (you). In لي (for me), we attach the pronoun ـي (me). Notice that the preposition is pronounced with an "a" sound (la) in لك, but changes to an "i" sound (li) in لي to easily blend with the "ee" sound of the attached pronoun.

Why do we use هو (he) to say "it is for me"?

Arabic does not have a neutral "it" pronoun. Every noun is either masculine or feminine. Because الطابع (the stamp) is a masculine noun, we must use the masculine pronoun هو (he) to refer back to it. In English, we naturally translate this as "it".

Why does the Arabic say هذا الطابع (this the stamp) instead of just هذا طابع?

In Arabic, if you want to say "this stamp" as a single descriptive phrase, the noun following "this" must be definite, meaning it needs the prefix الـ (the). If you drop the الـ and say هذا طابع, you create a full sentence that means "This is a stamp."

Does the pronunciation of لك change if I am asking a woman instead of a man?

Yes! The spelling stays exactly the same, but you pronounce it "laka" when speaking to a male, and "laki" when speaking to a female.