Why can't you put 用言 before だ, but you can put 用言 before だろう?


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?




It is regarded as a special case, from
[デジタル大辞泉](https://dictionary.goo.ne.jp/word/%E3%81%A0/#jn-132198)

> 「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](https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1392152628),
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.

