GrammarJLPT N5
→ explanatory particle
Explanatory / Reasoning Sentence Ending
JLPT N5Sentence-ending expressionCasual spoken
んだ (the casual contraction of のだ) is a sentence-ending expression that adds an explanatory or contextualizing nuance to a statement or question. When used in statements, it signals that the speaker is providing background explanation, giving a reason, or presenting information as connected to something already known or observed — essentially saying 'the thing is...' or 'it's that...'. When used in questions, it conveys that the speaker expects an explanation or is seeking the underlying reason behind something they have noticed, as in 'is it that...?' or 'so what happened is...?'. This distinguishes it from a plain statement or question, which simply states or asks about a fact without implying any contextual link. んだ is the casual spoken form; in polite speech it becomes んです, and in more formal or written contexts it appears as のだ or のです. Learners sometimes confuse んだ with から or ので, but while those particles explicitly mark a clause as a reason, んだ wraps the entire sentence in an explanatory tone, implying that the statement itself is the explanation or context for something.
Functions
#1 Providing an explanation or reason
Structure
Statement (plain form) + んだ
きのう 学校を 休んだの？ — うん、頭が 痛かったんだ。
Here んだ is used to provide the underlying reason for being absent. The speaker is not merely stating that their head hurt; by adding んだ, they frame the sentence as an explanation that answers the listener's implicit or explicit question about why they were absent. Without んだ, the sentence would just be a plain statement of fact with no explanatory connection to the previous context.You were absent from school yesterday? — Yeah, (the reason is) I had a headache.
Structures
- Verb (plain form)
- Verb (plain form) + んだ
- い-Adjective
- い-Adjective + んだ
- な-Adjective
- な-Adjective + な + んだ
- Noun
- Noun + な + んだ
Common mistakes
A very common mistake is using んだ after the polite ます-form, such as saying 食べますんだ, which is incorrect; んだ always attaches to the plain form. Another frequent error is forgetting the な before んだ with nouns and な-adjectives — saying 学生んだ or 好きんだ instead of the correct 学生なんだ and 好きなんだ. Learners also sometimes overuse んだ in every sentence, not realizing that it should only be used when there is an explanatory or contextual nuance; using it on plain factual statements with no implied connection to a broader context can sound unnatural or even confrontational. Finally, some learners confuse んだ with から or ので; while all three can relate to reasons, から and ので connect two clauses explicitly as cause and result, whereas んだ frames a single sentence as an explanation within the conversational context.
Related
のだのですんですからのでわけだ