～のだ
it is that / explanation
～のだ attaches to the plain form of a sentence and frames it as an explanation of the current situation. In polite speech it becomes のです, and in everyday conversation it is usually contracted to んだ or んです.
English has no exact equivalent, but it feels like “it's that ...” or “the thing is ...”: the sentence answers a question that is hanging in the air.
1. Explaining a situation
Use ～んです when your statement explains something the listener can see or is wondering about, for example, why you look unwell.
頭が痛いんです。
“It's that I have a headache.”
- 頭（あたま）= head
- が = subject marker
- 痛い（いたい）= hurts, painful
- んです = explanatory ending (spoken form of のです)
Plain 頭が痛いです simply reports a fact. With んです, the same sentence says “and that explains what you're seeing”.
2. Asking for an explanation (～んですか)
In a question, ～んですか asks the listener to explain something you noticed.
どうして遅れたんですか。
“Why were you late?”
- どうして = why
- 遅れた（おくれた）= was late (past form of 遅れる, to be late)
- んですか = question asking for an explanation
How to form it
Plain form + のだ／んだ (polite: のです／んです; na-adjectives and nouns take な)
- 痛い（いたい）= painful → 痛いんです（いたいんです）= (it's that) it hurts
- 行く（いく）= to go → 行くんです（いくんです）= (the thing is) I'm going
- 学生（がくせい）= student → 学生なんです（がくせいなんです）= (it's that) I'm a student
Key idea
～のだ connects your sentence to the situation around it. Use it when your words answer an unspoken “why?”, yours or the listener's.
Other N4 grammar
Updated .