んだ・んです
Explanatory, Emphasis, The fact is
Structure
［い］Adjective + ん(1) + だ
［な］Adjective + な + ん(1) + だ
Noun + な + ん(1) + だ
(1) の
Explanation
のです is a grammar pattern that comes in several forms which all carry basically the same meaning. This expression functions similarly to standalone だ and です (which state that something is a fact). However, the addition of の serves to emphasize the connection being made. Because of this, it often carries the nuance of, 'The reality is that (A)' in English.
- 今日は寒いんです。It's just that it's cold today. (Strong assertion)
- 彼が正しいのです。He is the one who is correct. (Strong assertion)
In these sentences, the ん or の can be thought of as simply adding extra weight to the statement. Since の contributes this layer of emphasis, this pattern is used very often when providing explanations or reasons.
Caution
There is no meaningful difference between ん and の. While の is technically the more 'proper' Japanese, ん emerged as a contraction to make pronunciation smoother. In modern spoken Japanese, ん is by far the most common form, and using の can sound somewhat formal or even a little stiff.
While the だ and です versions are both equally natural on their own, only the です form is used when adding か to ask a question, ～んですか. This structure indicates that the speaker is seeking an explanation for something they have observed.
- 犬が好きなんですか？So, you like dogs? (Is that why you're petting one?)
- 何かあったのですか？Did something happen? (You seem upset.)
Examples
寒いんですか。
Is it cold?
少し眠いんです。
I'm a little sleepy.
電車に乗るんです。
I'm taking the train.
何を飲むのですか。
What are you going to drink?
お腹が空いたんです。
I'm hungry.
この料理は辛いのですか。
Is this dish spicy?
今日はとても楽しいのです。
Today is really fun.
毎日、運動するのです。
I exercise every day.
黒がいいのですか。
Is the black one okay?
雨が降っているんです。
It's raining.
とても嬉しいんです。
I'm so happy.
道が分からないのです。
I don't know the way.