How does あるのですか work in this gramatically?


A Japanese friend wrote to me: なぜsomethingsomethingに興味あるのですか。

  1. Shouldn't she have used が before ある?
  2. Could you just say ありますか instead of あるのですか?




「なぜ～に興味あるのですか」 is a perfectly valid sentence, if not very formal.

  1. In formal sentences, yes, が is necessary here. In casual sentences (particularly in speech), particles are often omitted. See: [What are the guidelines of omitting particles?](https://japanese.stackexchange.com/q/3151/5010)
  2. This の is almost mandatory because this sentence is [seeking an explanation based on the previous context](https://www.wasabi-jpn.com/japanese-grammar/question-markers/) (which is of course "you are interested in something"). So saying ありますか instead of あるのですか would sound awfully unnatural. Basically の (or ん) is almost always used when you ask a reason using なぜ/どうして.




「なぜ～に興味あるのですか」 is a perfectly valid sentence, if not very formal.

  1. In formal sentences, yes, が is necessary here. In casual sentences (particularly in speech), particles are often omitted. See: [What are the guidelines of omitting particles?](https://japanese.stackexchange.com/q/3151/5010)
  2. This の is almost mandatory because this sentence is [seeking an explanation based on the previous context](https://www.wasabi-jpn.com/japanese-grammar/question-markers/) (which is of course "you are interested in something"). So saying ありますか instead of あるのですか would sound awfully unnatural. Basically の (or ん) is almost always used when you ask a reason using なぜ/どうして.

