My textbook ( "Elementary Korean", Tuttle) features the question (where N is a noun)
이것은 N 입니까?
along with the following two answers:
네, (그것은) N 입니다
as the translation of "Yes, that is N", as well as
아니요, (그것은)N 이/가 아닙니다.
as the translation for "No, that is not N".
Why do I need the subject particle when negating the sentence, but not when just stating it? It's probably not a big deal but it really bothers me not to know why they chose to use the subject particle only in the second sentence.