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I look up dictionary which says だろう is 未然形 of だ plus う。 So I think that だろう should have the same 接続 pattern with だ, but it doesn't.

Acceptable:

彼は会社を休むだろう。

Unacceptable:

彼は会社を休むだ。

Why?

1 Answer 1

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It is regarded as a special case, from デジタル大辞泉

「だ」の未然形・仮定形は、動詞・形容詞・助動詞「れる・られる・せる・させる・た・たい・ない・ぬ・らしい」などの終止形にも付く。

So 「休むだろ(未然形)う」「休むなら(仮定形)ば」 are possible while だ(終止形) cannot follow 休む(終止形).


According to this chiebukuro answer, it is a phenomenon that stems from disambiguation: Originally う・よう meant both volition and future, then だろう became the standard for future and う・よう just for volition.

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