副詞 before copula


When reading through [The meaning of
そう](https://japanese.stackexchange.com/questions/25086/the-meaning-
of-%E3%81%9D%E3%81%86), I was thinking about そう's status as a 副詞. Certainly,
it acts like many other 副詞 in that it can directly modify verbs.

However, I was curious about the usage そうだ -- a form that I _think_ not all 副詞
support.

**副詞 that directly work with だ:**

こうだ、どうだ、ああだ (?) all work, along with some other things classified as 副詞, like
なぜだ.

**副詞 that work with だ by omission:**

Then, I think there are other 副詞 that work, like きっとだ, たまにだ. But, I think
these only work if you interpret something as being omitted, like きっと（できるの）だ
or たまに（するの）だ, for example.

In other words, semantically, the 副詞 does not seem to be modifying the meaning
of the copula.

**副詞 that don't work with だ at all:**

×決してだ, ×どうかだ, and probably more. These seem to share the trait that they need
specific endings (決してする **な** , どうかして **ください** ) which the だ gets in the way
of.

**Question:**

Assuming my judgments are correct, this seems like somewhere where the 学校文法
framework is failing to explain what's going on fully. Is there a better way
to think about these things? Are こう、そう、どう、なぜ actually also nouns? What is the
actual syntax/theory of the cases where 副詞 work by omission?




I don't know if this will answer your question but I hope this helps. I will
explain it in a different way. If this is not what you are looking for, I
apologize ahead of time.

Japanese 副詞 can be broken into several groups. I do not know the English names
so I will write them as I know.

情態副詞： ゆっくり、きらきら、てくてく、ぼんやり etc. They express the state of something.  
程度副詞： とても、もっと、非常に etc. They express things like "very" big instead of just
big.  
陳述{ちんじゅつ}副詞： きっと～だ/だろう、たぶん～だろう、決して～（し）ない etc.

These adverbs usually express the speaker's feelings and are also more than
likely used in a set, rather than stand alone. But since it is common to omit
information, you may not notice the patterns, especially with words like たぶん
and きっと. Most of these 副詞 are set up with either a negative or だろう. If the 副詞
is used with だ, it is basically stressing the message of the sentence.
あなたはきっとできるだろう and あなたはきっとできるんだ（or even きっとできるよ etc.) is changing from "I think
you can definitely do it" to "I'm sure you can do it" kind of nuance. I hope
that makes sense. Another example is with negative 陳述副詞 like 決して～（し）ない. With
彼は決してうそをつかない and 彼は決してうそをつかないだろう, the former shows more confidence in the
speaker's assertion.

However, こうだ、そうだ、ああだ、どうだ are known as 指示詞. They only point out
directions/distance from object(s) A and B, either physically or
psychologically.

So, I think you can see that こさあど words are not considered 副詞 but as 指示詞
instead.

