What is the meaning of ～んです/～のだ/etc?

Question

This sentence ending has started to creep up more and more in my current Japanese classes, and I'm still a bit unsure what all it can mean, how to use it, or even when I should think to use it. My Japanese teacher in a previous class mentioned that it can be used to "give emotion" to a statement or to inform the listener of new knowledge. Something along those lines. But I'd love a more formal explanation.

1. What does it mean?
2. Is there a particular usage pattern to follow?
3. What are some good examples of the various uses of this pattern, and how would they compare to the same sentences, only without んです?

Answer 1

のだ (んだ, のです, んです, or の) seems to create an information deficit. There was an article written by Derek Schaab about this.

The の here is called a formal noun. It is very often translated to "It is that~", "the case", "the situation" or "the fact". When used to end a sentence, it may imply that it is an explanation to some previous context (filling up the information deficit). When used to ask a question it begs a deeper explanation over a superficial reply (creating an information deficit).

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Consider:

1. 私は猫が好きだ - I like cats

2. 私は猫が好きなのです。 - It is that I like cats

For 1, it is a neutral statement that just says that you like cats.

For 2, it fills in a blank with the fact that you like cats as some sort of explanation. For example, if you got distracted by a cat and went out of your way to pet the cat, and your friend who was with you at the time gave you a quizzical look. You may reply 私は猫が好きなのです。 as an explanation to your behaviour to fill in the information deficit of your friend.

You can also use のです to immediately fill in an information deficit that you think the listener should have:

• 今日は出かけられない。宿題がたくさんあるのです。 - "I cannot go out today. (The reason being that) I have lots of homework.

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のです can also be used when no information is shared by the speaker and listener, and is not used to express reason or an explanation. When used as such, the speaker speaks as though the information was shared and the effect this creates is to involve the listener in what the speaker is talking about, or to create an emphasis on what the speaker is talking about (by means of creating a perception of information deficit of the listener).

(heefske has done a better job of explaining this in another question the summary is that it builds rapport by showing emotional investment in the issue.)

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Usage pattern:

• Verb + のです

  するのです - it is that ~ is/will be done

• Verb(past) + のです

  したのです - it is that ~ was done

• い-Adjective + のです

  高いのです - it is that it is expensive

• い-Adjective(past) + のです

  高かったのです - it is that it was expensive

• Noun + な(attributive だ) + のです

  猫なのです - it is that it is a cat

• Noun + だった(past だ) + のです

  猫だったのです - it is that it was a cat

• Noun(adjective) + な(attributive だ) + のです

  静かなのです - it is that it is quiet

• Noun(adjective) +　だった(past だ) + のです

  静かだったのです - it is that it was quiet

Answer 2

～んです is a contraction of ～のです, which in turn is the polite form of the plain/informal ～のだ (which is also seen sometimes as ～んだ.)

～のです/～のだ are used to emphasize the previous statement. According to Daijisen and Daijirin, のです emphasizes explanations of cause/reason/basis etc. In the forms of ～んですか/～のですか, it can create an emphasized or even demanding question.

See also the sci.lang.japan FAQ for more information.

Answer 3

One usage that I was taught and is not mentioned above for んです is to soften a question or statement.

For example

レストランに　行きたいです I want to go to a restaurant.

is a bit demanding or matter-of-fact. Changing to 行きたいんです softens this. Imagine a friend asks you about what you both should do for lunch. 行きたい sounds a bit forceful, like you will consider nothing else. The addition of ん softens it.

Consider the following:

あなたはなぜトルクに　行きたいですか？ Why do you want to go to Turkey?

あなたはなぜトルクに　行きたいんですか？ Why do you want to go to Turkey?

In the first example it could be somebody at immigration or other official quizzing you. In the second it could be a friend asking about your planned holiday.

Answer 4

んだ is a spoken form of のだ. There is actually an exact entry for のだ in the Japanese dictionary that comes with MacOS/iOS (スーパー大辞林 / Super Daijirin Japanese Dictionary); I think the explanations make quite some sense:

の◦だ

（連語）〔準体助詞「の」に断定の助動詞「だ」の付いたもの。話し言葉では「んだ」「のです」となることも多い〕

① 原因・理由・根拠などの説明を強く述べる。「こうして独り住まいをしていると，ますます孤立感が深まっていく―◦だ」「弟の言うことを聞けば聞くほど，気の毒でしかたがなかった―◦だ」

② 意志的な動作を表す語に付いて，その動作主の決意や相手に対する要求などを表す。「なんとしても，この事だけは達成する―◦だ」「早くここから出て行くんだ」

③ （「のだった」の形で）事態の説明をやや詠嘆的に言い表す。「一人思い出にふける―◦だった」

So it could be that the speaker is trying to

1. put an emphasis to their narration.
2. express a strong will or a demand to the other side. (I think this also covers the example mentioned in VictorySaber's answer.)
3. (のだった）express slight exclamation at their narration of an event.

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There is also a separate entry for なのだ though it seems to more or less correspond to the first meaning of the entry above.

なの◦だ

（連語）〔形容動詞の語尾または助動詞「だ」の連体形「な」に助詞「の」が付き，さらに断定の助動詞「だ」の付いたもの。話し言葉では「なんだ」となることが多い〕

強い断定を表す。「これが僕のやりたいこと―◦だ」「試合結果がよくなかったのは，何よりも練習時間が少なかったから―◦だ」〔「だ」の活用に応じて，「なのだろう」「なのだった」「なのではないか」などの形でも用いられる〕

Answer 5

助動詞「だ」の連体形「な」に準体助詞「の」が付いたもの 上昇調のイントネーションを伴って、質問の意を表す。 https://kotobank.jp/word/%E3%81%AA%E3%81%AE-347401