Questions & Answers about Nie wiem, dlaczego on ma mój numer.
Why do we use wiem instead of znam for "I know" here?
Wiem comes from wiedzieć, which is used when you know a fact or before question words like dlaczego (why). You would use znam (from znać) if you directly knew a noun, like a person, place, or thing.
Is the comma before dlaczego required?
Yes! In Polish, you must place a comma before a new clause (a part of the sentence with its own verb). Whenever you connect a main sentence to a question word like dlaczego, a comma is strictly required.
What case is mój numer in, and why didn't its ending change?
It is in the Accusative case because it is the direct object of the verb ma (has). Because numer is a masculine inanimate noun (a thing, not a person or animal), its Accusative form looks exactly like its basic dictionary form. The word mój matches it, so it also doesn't change.
Could I drop the word on and just say dlaczego ma mój numer?
You can, but it might be unclear without context. While Polish usually drops pronouns like ja (I) or ty (you), the third-person verb ma can mean he has, she has, or it has. Including on makes it completely clear you mean "he".