Breakdown of Dlaczego nosi mój zegarek, a nie swój?
nie
no / not
a
and / whereas (contrast)
mój
my
dlaczego
why
Questions & Answers about Dlaczego nosi mój zegarek, a nie swój?
Why do we use swój here instead of jego (his)?
The pronoun swój means "one's own" and always refers back to the subject of the sentence. Since the subject is "he" (implied by the verb) and the watch belongs to him, you must use swój. If you said jego zegarek, it would mean he is wearing another man's watch!
What case are mój and swój in here, and why haven't their endings changed?
They are in the Accusative case because they are the direct object of the verb nosić (to wear). The word zegarek (watch) is a masculine inanimate noun. For masculine inanimate nouns, the Accusative form is identical to the basic Nominative form, so mój and swój don't need any special endings here.
How do we know the sentence means "is he wearing" if there's no word for "he"?
The verb nosi is in the third-person singular form. In Polish, subject pronouns like on (he) or ona (she) are usually dropped because the verb ending already tells you who is acting. Without extra context, this sentence could equally mean "Why is she wearing my watch, and not her own?".
Why does the sentence use a nie instead of i nie for "and not"?