"Why is this answer incorrect?"
'んです' is popular than 'のです' in the conversation. But 'のです' is original correct form. You can confirm about it on the plural websites, I think.
one is :
http://maggiesensei.com/2010/09/08/request-lesson-when-and-how-to-use-%E3%82%93n-%E3%81%AE%E3%81%A7%E3%81%99%E2%86%92%E3%82%93%E3%81%A7%E3%81%99%EF%BC%89/ (But sorry, I don't read all of this.)
If you agree me, report it.
That's right, the version with んです or のです indicate a reason for sth and sounds more explanatory than simply "x です".
For an example someone could ask you "Why don't you buy it?" and then you will ask him "お金がないのです" (because I don't have money) or more conversational "お金がないんです" (what means the same, but makes it sound more casual) It is worth meaning that you can also say "お金がないんだ" which is even more casual than previous one.
One important thing - this explanation is only my own understanding of the matter and is based on maggiesensei page(he makes it even more clear than me :3). So thank you!
んです is used to indicate reason When you use it in a question, you're sort of asking for a more elaborate explanation than just asking "why"
-ndesu is another version of "no desu" のです - Expression 1. the expectation is that ...; the reason is that ...; the fact is that ...; it is that ... (Other forms: のだ、んです、んだ)
so, "この答はどうして違うんですか" would be like asking "What is the reason this answer is wrong?"
Hope that clears things up :)
It's a contraction of のです, where the の turns what came before it into a noun phrase. You can think of の here as meaning "the one" or "the thing". As in "it's a different one".
yes. 1 - この答どうして違うんですか "This answer, why is it wrong"
2 - どうしてこの答は違うんですか "Why is this answer wrong?"
3 - どうして違うんですか、この答 "Why is it wrong; this answer?" - more verbal and informal
The thing with Japanese is that as long as you put the correct particles with the right element of the sentence (Nouns, verbs, adverbs, etc) you can structure the sentence almost in any way you want.
the んです is used here, because you are asking for a reason the answer is wrong.
この答どうして違うんですか is basically "This answer, (please explain) why is it wrong?" [I think it would have been easier to understand if they had written it この答、どうして違うんですか)
To me, it would have made more sense to structure the sentence like どうしてこの答は違うんですか
then you can see it like どうして「この答は違う」んです indicating that someone has said "this answer is wrong", and you ask them to explain why.