How to say "Whose hat is this?" in Japanese
If I ask ‘whose hat is this?’ in Japanese

Can I say:

このぼおしはだれですか

or does it have to be

これはだれのぼしですか

Question

How to say "Whose hat is this?" in Japanese
If I ask ‘whose hat is this?’ in Japanese

Can I say:

このぼおしはだれですか

or does it have to be

これはだれのぼしですか

Answer

「これはだれのぼうしですか」 is right.

Answer

The 2nd one. The first one is asking, "who is the hat?"

Answer

Just more info. You have to write ぼうし but it's often pronounced ぼおし. It's confusing for learners. I'll give you another example. がっこう（school). it's pronounced がっこお。Boy's name、たろうis pronounced like たろお。

これは  誰の（だれの）帽子（ぼうし）ですかis correct. it's not かばん（bag).   Sayaka's answer is right. And you cay say, このぼうしは、だれの（だれのもの）ですか too.

Answer

この帽子は誰のですか? 
or
これは誰のかばんですか?

good luck!