Q. What is "~n desu" which is sometimes used at the end of a sentence?
A. This expression is used quite often in conversation, though it might be difficult to use correctly for beginners. It has an explanatory or confirmatory function. The difference between "~masu" (a normal ending of the verb) and "~n desu" is very subtle, therefore it is very hard to translate the nuance. It can be translated as "It is the case that ~" or "It is for the reason that ~". However there is no English equivalent.
Compare these sentences.
(1) Ryokou ni iku n desu ka. (2) Ryokou ni ikimasu ka.
For the sentence (1), she assumes that he is going on a trip, and wants him to confirm it. On the other hand, in the sentence (2), she simply wants to know if he is going on a trip or not.
"~n desu" is attached directly to a plain form of the verb. In the informal situation, "~n da" is used instead of "~n desu".
|Ashita doubutsuen ni ikimasu.
|I am going to the zoo tomorrow.
|Ashita doubutsuen ni iku.
|Ashita doubutsuen ni iku n desu.
I am going to the zoo tomorrow.
|Ashita doubutsuen ni iku n da.
"Why" questions are often asked with this expression, because they are asking for a reason or an explanation.
|Doushite byouin ni iku n desu ka.
Haha ga byouki nan desu.
|Why are you going to the hospital?
Because my mother is sick.
|Doushite tabenai n desu ka.
Onaka ga suite inai n desu.
|Why don't you eat?
Because I am not hungry.
Click here for another usage for "~n desu".