Sentence ending : N desu

Examples
1. A : Kaoiro ga warui desu ne.
"You are pale, aren't you?"(Lit.)
2. B : Ee, atama ga itai n desu.
"Yes, (because) I have a headache."
3. A : Sore wa nan no hon desu ka.
"What is that book about?"
B : Nihongo no hon desu. Omoshiroi n desu.
"It's a Japanese book. (I am absorbed in reading it, because) It's interesting."


The conversation in Example 1 starts with A's recognition that B looks unusually pale and A says "You are pale."and B knows that he is pale, so he gives the explanation for it.
The conversation in Example 2 starts with A's recognition that B is unusually absorbed in a book and A is interested in what kind of book B is reading. A asks "What is that book about?" B answers A's question and after that B notices why A asked such a question, so he gives an explanation for that by saying"It is an interesting book."
In both examples, it is inadequate to say itai desu or omoshiroi desu instead of itai n desu or omoshiroi n desu. The basic meaning of a sentence does not change by the addition of n desu to it.
However, the presence of n desu adds certain overtones to the statement, for it indicates some explanation, either of what was said or done, or of what will be said or done.
N desu is preceded by the non-polite form of a verb or an -i adjective, by a –na adjective ending in na, or by a noun { na. Here, only the non-polite imperfective affirmative form of an -i adjective is introduced.
Note that n in n desu receives the timing of a full syllable.
Sentences ending in n desu ka are not used in asking questions to start a conversation unless there is an unusual situation. For example, if you ask someone Isogashii desu ka. "Are you busy?"
you are simply asking whether he is busy or not. But if you ask
Isogashii n desu ka.
it can mean"Do you look tired because you are so busy?", "Are you so busy that you can't come with us to have a drink?"and so forth depending on the situation.
Thus questions ending in n desu ka can imply various emotions such as concern, surprise, irritation or criticism.