indicates reason or excuse
Sentence +
Used at the end of a sentence to express a reason or excuse, often in a slightly emotive or defensive way. It treats the reason as an objective fact. In casual speech, it often changes to
Noun / な-Adjective + な +
When used with Nouns or な-Adjectives in the present tense, な is required before
お腹が空いているんだ
Because I'm hungry.
野菜が嫌いなんだ
Because I don't like vegetables.
Verb / い-Adjective +
Verbs and い-Adjectives can directly precede
高かった
Because it was expensive.
もう食べたんだ
Because I already ate.
This grammar point is often used by children or women in casual conversation to sound a bit cute or to emphasize a personal reason. When used with な or ね at the end, it adds a sense of seeking empathy or agreement.