のだ

also: んだ

Meanings

explanatory/emphatic sentence-ending expression

Adds explanatory, emphatic, or reasoning nuance to a statement. Often used when providing a reason, clarifying a situation, or asserting a fact. The の/ん adds emphasis. In questions, it seeks an explanation. Equivalent to のです in more formal contexts.

Related words

なのだ
のだ attaches after verbs and i-adjectives to give the same explanatory/assertive nuance that なのだ gives after nouns and na-adjectives.
ねん
のだ is the standard Japanese explanatory sentence-ending, while ねん is its Kansai dialect equivalent with a more casual, assertive tone.
やん
やん functions as a Kansai-dialect emphatic sentence-ender similar in role to the standard explanatory ending のだ.

Politeness

のですpolite 丁寧語
のです is the more formal, written-register counterpart of のだ／んだ, which is used in casual spoken Japanese.

Don't confuse with

からN5
から explicitly states a reason or cause, while のだ adds explanatory or emphatic nuance to a statement without necessarily giving a reason.

Etymology

Combination of the nominalizer の (or its contracted form ん) and the copula だ. The の nominalizes the preceding clause, and だ asserts it, creating an explanatory or emphatic effect.