のだ (explanatory)

• Reading: のだ
• Romaji: noda
• Formation: Sentence (plain form) + のだ/のです (casual: んだ/んです)

Meaning

A sentence ending that indicates the speaker is explaining a reason, providing background information, or seeking an explanation from the listener. In polite speech it appears as のです, and in casual speech it commonly contracts to んだ or んです.

のだ/のです is one of the most important structures in Japanese. It signals that the speaker is not simply stating a fact but framing it as an explanation or background for something. For instance, if someone asks why you look tired, responding with 昨日寝なかったんです means 'The thing is, I didn't sleep last night.' It can also seek explanation: どうしたんですか means 'What happened?' — asking for the story behind an observed situation. The casual contractions んだ and んです are far more common in daily speech than the full のだ/のです, which can sound stiff. Learners often overuse のだ where a simple statement suffices, or underuse it when context clearly calls for explanation. A helpful test: if 'the thing is' or 'it's that' fits naturally in an English translation, のだ is likely appropriate. Before nouns and な-adjectives, な must be inserted: 学生なんです, 静かなんです.

Examples

1. 実は来週引っ越すんです。 Actually, I'm moving next week.
2. 電車が遅れたので、遅刻したんです。 The train was delayed, so I was late.
3. 昨日から熱があるんだ。 I've had a fever since yesterday.

Usage Guide

Context: spoken, written, everyday

Tone: explanatory

Do Say

• 今朝は朝ごはんを食べなかったんです。
• すみません、道に迷ったんです。
• あの映画、もう三回見たんだよ。
• どうしてそんなに嬉しそうなんですか。

Don't Say

• はじめまして、田中なんです。(Using なんです in a simple self-introduction — のだ implies an explanation but none is called for here) → はじめまして、田中です。
• 明日は休みのだ。(Inserting のだ after a noun without な — nouns require なのだ) → 明日は休みなんだ。
• 明日雨が降りますんです。(Attaching んです to polite ます form — it must follow the plain form) → 明日雨が降るんです。

Origin & History

のだ developed from the nominaliser の plus the copula だ. By nominalising the preceding clause and then asserting it with the copula, the speaker frames the statement as background explanation. This construction was well established by the Meiji period.

Cultural Context

Generation: All ages

Social background: Universal