-n-desu ( Genki I Chapter 12)

~んです

There are two ways in Japanese to make a statement. One is to simply report it, and the other is offered as a means of explanation. んです follows the short form of a predicate, and the predicate can be in the affirmative, negative, past, or non-past. When it follows a noun or a な-adjective, な comes in between.

あしたテストがあるんです。  I have an exam tomorrow (so I can’t go out tonight.)

トイレにいきたいんです。   I have to go to the bathroom (so tell me where it is.)

~んです can be used in questions to invite explanations and further clarifications. It is often used with question words such as どうして and どうした.

Q: どうしてかれとわかれたんですか。

Why did you break up with your boyfriend? (You’ve got to tell me.)

A: かれ、ぜんぜんおふろにはいらないんです。

Oh, him. He never takes a bath. (That’s a good enough reason, isn’t it?)