What is the meaning of ～んです/～のだ/etc?


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?




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](http://sakamotomaaya.com/291/information-deficits).

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).

* * *

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.

* * *

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](https://japanese.stackexchange.com/a/3685/542) the summary is that
it builds rapport by showing emotional investment in the issue.)

* * *

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




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](http://sakamotomaaya.com/291/information-deficits).

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).

* * *

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.

* * *

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](https://japanese.stackexchange.com/a/3685/542) the summary is that
it builds rapport by showing emotional investment in the issue.)

* * *

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

