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Grammar I - 12.01
 ~んです - ~ n desu
 

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 - n desu follows the short form of a predicate, and the predicate can be in the affirmative, negative, past, or non-past.

   
  A report is an isolated description of a fact. When you are late for an appointment, you can already report in Japanese what has happened,

バス が 来ません でした。

The bus didn't come.
basu ga kimasen deshita.  

 

This sentence, however, does not have the right apologetic tone, because it is not offered as an explanation for anything.
If you want to mention the busses failing to run on time as an excuse for being late, you will need to use the explanation mode of speech, and say:
バス が 来なかった ん です。 (As it happens), the bus didn't come.
basu ga konakatta n desu.  
   
あした テスト が あります。 I have an exam tomorrow (a simple observation).

ashita, tesuto ga arimasu.

 
あした テスト が ある ん です。 I have an exam tomorrow (so I can't go out tonight.)

ashita, tesuto ga aru n desu.

 
   
   
トイレ  いきたい です。

I want to go to the bathroom (declaration of ones' wish).

toire ni ikitai desu.  
トイレ  いきたい  です。 I have to go to the bathroom (so tell me where it is.)
toire ni ikitai n desu.  
   
  n desu goes after the short form of a predicate.
 

The predicate can be either in the affirmative or in the negative, either in the present tense or in the past tense.
n desu itself is invariant and does not usually appear in the negative or the past tense forms.
In writing, it is more common to find no desu instead of n desu.

  In casual exchanges, n desu appears in its short form, n da.
In casual questions, n desu ka is replaced by no. We will examine these further in Lesson 15.
   
成績が よくない ん です。 (As a matter of fact) My grade is not good.
 (in response to the question. "Why do you look so upset?")

seiseki-ga yoku-nai n desu.

 
試験が 終わった ん です。 The exam is over. (That's why I'm smiling.)
(explaining to a person who has caught you smiling)
shiken-ga owatta n desu.  

 

 
  When it follows a noun or a な-adjective, な comes in between.
 

na-adjective

  Reported sentences:
静か です It's calm

shizuka desu

 
静かな ん です Because it's calm (as explanation of a fact)

shizuka-na n desu

 
 

Noun:

   
学生 です。 It's a student.
gakusei desu.  
学生 な ん です。 Because he's a student (that's why),
gakusei na n desu.  
 

~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 - doo shite (why) and どうした - doo shita (what has happened).
   
Q: どうして 彼 と 別れた ん です か。 Why did you break up with your boyfriend? (You've got to tell me.)

doo-shite kare to wakareta n desu ka?

 
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?)

kare, zen-zen ofuro ni haira-nai n desu.

 
   
   
   
どうした ん です か? What Happened? (You look shattered)
doo-shita n desu ka?  
猫が 死んだ ん です。 My cat died. (That should explain how I look today.)
neko-ga shinda n desu.  
   
  You can also use n desu to provide an additional comment on what has just been said.
とても いい 教科書 です ね。 That's a great textbook that you are using.
totemo ii kyookasho desu ne.  
ええ。
私の 大学の 先生が 書いた ん です。
You bet.
The professors at my university wrote it (for your information).
ee.
watashi-no daigaku-no sensei-ga kaita n desu.