modality N4 essential casualpolitewritten

のだ — explanatory のだ / んだ

のだ

Meaning

it is that ~ / the reason is that ~ — frames a statement as an explanation, reason, or background, not a bare fact

のだ ties what you say to the situation: it signals 'here's the explanation / the deal', rather than just reporting. 道みちが混こんでいた is a neutral 'traffic was heavy'; 道みちが混こんでいたんだ offers it as the reason for something (why you're late). Adding it where nothing is being explained sounds heavy or insistent.

Key sentence

顔色かおいろが悪わるいね。— 頭あたまが痛いたいんだ。

You look pale. — It's that I have a headache.

Formation

Attaches to	Form	Example
Verb / i-adjective (plain form)	plain + のだ	行いく → 行いくのだ / 痛いたい → 痛いたいのだ
Noun / na-adjective (plain)	N/na-adj + なのだ	学生がくせい → 学生がくせいなのだ / 静しずか → 静しずかなのだ

When: のだ is the plain/written form; everyday speech contracts it to んだ, and the polite forms are んです / のです.

Variants

んだ — the spoken contraction of のだ: 痛いたいんだ. んです / のです — the polite forms — see those nodes.

Examples

どうして泣ないているの。

Why are you crying? (asking for the explanation behind it)

今日きょうは祝日しゅくじつなんだ。

(The thing is,) today is a holiday.

電車でんしゃが止とまって、道みちに迷まよったんだ。

The train stopped and I got lost — that's what happened.

When you can't use it

Don't attach のだ/んです to every sentence. A plain report of fact takes no のだ; のだ implies there is an explanation or reason behind the statement. Over-using んです is a classic learner habit that makes neutral statements sound loaded.

Easily confused with

んですんです is just the polite-spoken realization of this same のだ; のだ/んだ are the plain forms. 〜からから states the reason explicitly ('because ~'); のだ frames the whole statement as the explanation without a 'because' word — often the answer to an unspoken 'why?'.

See のだ in real sentences

Jengo shows のだ the way you actually meet it: inside real Japanese sentences, so it sticks instead of staying an abstract rule.

Study it in Jengo

Sources Compiled with reference to A Dictionary of Advanced Japanese Grammar.

← Back to the grammar tree