Is ending a sentence with the plain form grammatically correct?


I am far into intermediate level at this point, but I still have not
conclusively found an answer to this. The response that I typically receive is
that it's "casual speech," but that doesn't tell me anything about correctness
(In English, except for contractions, informal speech is not correct). If you
were to ask a university professor of Japanese or a linguist what is
grammatically correct, would they say that ending with the plain form of a
verb is technically grammatically correct? What about「だ」、「んだ」、and「の」？




Of course a sentence like 鳥は飛ぶ ("Birds fly.") is 100% grammatically correct.
It's 100% standard, too; no slang/dialect is concerned. I'm not an expert, but
you never need a professor nor a linguist to judge the correctness of such a
simple sentence. Don't be that skeptical about your textbook.

But did your textbook really call this **casual** style? I think textbooks
usually call this **plain** style (as opposed to **polite** style sentences
typically ending with ます/です). This style is used both in formal and casual
settings as long as politeness is not necessary. For example, Japanese
Wikipedia has literally millions of sentences ending with the plain form of a
verb.

Likewise, a sentence ending with だ is 100% grammatically correct. It's one of
the most basic word (copula) that corresponds to English "be", after all. It's
less common in Wikipedia because である is preferred there, but that's another
story. If grammatical correctness is your concern, it's unequivocally correct.
The same for のだ.

As for の, are you referring to the [explanatory-の used as a question
marker](https://www.wasabi-jpn.com/japanese-grammar/question-markers/)? Then
of course that's 100% grammatically correct, too. This one is indeed casual
and colloquial.

