副詞 before copula

Question

When reading through The meaning of そう, 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?

Answer 1

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.