Questions & Answers about Gdzie jest mój kapelusz?
Why do we use mój instead of moja or moje?
Why doesn't the word kapelusz change its ending here?
Because it is the subject of the sentence. When you are simply asking about the location of an object using jest (is), the noun stays in its basic dictionary form, which is the nominative case.
Is it okay to drop the word jest in spoken Polish?
Yes, in very casual, everyday speech, you might hear Polish speakers simply say Gdzie mój kapelusz? without the verb. However, keeping jest is the standard and most correct way to ask.