What is 人 in 人だ (gramatically)?


In the elementary sentence:

> 人だ

人 is not marked with a particle. So what does it function as, gramatically?

If we translate the sentence as

> A person exists.

then presumably "person" functions as the subject of the sentence (so would
have to be marked by either が or は, no?). If we instead translate the sentence
(probably more idiomatically) as

> There is a person.

Then "there" functions as the sentence's subject, correct? So does that mean
there's a hidden pronoun in the Japanese sentence? Something like

> [あれが]人だ

If that's the case, what is 人, grammatically?




First, 人だ does not actually mean "a person exists". That would actually be
人がいる. 人だ actually says "(it) is a person" (that is, it is describing some
other thing as being a person).

だ/です ("is") is a bit special in Japanese in that unlike verbs (or unlike other
verbs, depending on whether you consider だ itself to actually be a verb or not
(opinions differ)), it always takes an argument immediately before it (without
any particle). This argument is generally either a noun or a な-adjective.

So in this sentence, 人 is a noun, and だ is the _copula_ (the verb/grammatical
construct which expresses equivalence), which always comes immediately
following the noun/な-adjective which it is saying something is.

As far as "part of speech" it could arguably be considered to be the "object"
of the copula (more formally referred to as a "predicate"), but unlike other
verbs the copula just does not use を to indicate its object, but instead just
immediately follows it. Alternately you could say that it has a part of speech
which doesn't have an equivalent term in English (I'm not sure if there is a
Japanese term for it or not)..

