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」?

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    In the first place informal speech being incorrect in English is not what any linguist would tell you
    – Angelos
    Commented Jan 13, 2021 at 2:20
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    I guess it would help if you clarified what you mean. If something is accepted by native speakers, that makes it grammatically correct in a descriptive sense. Japanese and English don't have organizations that prescribe correct grammar.
    – Leebo
    Commented Jan 13, 2021 at 2:33
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    I'm puzzled by this question... Any beginner textbook must have many examples of "ending a sentence with a plain form." Are you asking if simple sentences like 鳥は飛ぶ or 太陽は東から昇る are grammatically correct? Why do we need a professor to judge the correctness of such a simple sentence?
    – naruto
    Commented Jan 13, 2021 at 6:19
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    Style guides aren't trying to establish the one true variety of a language; they only prescribe how to write for publishers that adhere to them. 'I ain't tough' is not standard English, but that doesn't mean dialects where it is grammatical lack rules altogether. A speaker of such a dialect would not say 'I tough ain't' under normal circumstances. Also, what we view as 'standard language' is largely an accident of history - any dialect could have become the standard under the right circumstances. In Japanese itself, a Kansai-based dialect was formerly standard, and now it's a Kanto one.
    – Angelos
    Commented Jan 13, 2021 at 7:02
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    @AeonAkechi yes, I find the implication that something like Kansai-ben would be considered "grammatically incorrect" just because it wouldn't be used to write a term paper strange indeed.
    – Leebo
    Commented Jan 13, 2021 at 8:29

1 Answer 1


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? Then of course that's 100% grammatically correct, too. This one is indeed casual and colloquial.

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    – chocolate
    Commented Jan 13, 2021 at 14:20

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