adverbs - Part of speech of "very," "extremely," "really," and "quite"

While working on developing the lexicon in one of my constructed languages, I encountered a slight difficulty in using standard classifications for words like very, extremely, really, and quite.

To demonstrate this, here is an example sentence with a noun, an adjective, a verb, and an adverb:

The bad dog howled angrily.

If any of those parts of speech is replaced by a word like very, extremely, etc., the sentence does not seem to be grammatical. For example,

The extremely dog howled angrily. --(Extremely does not work as an adjective)

The bad very howled angrily. --(Very does not work as a noun)

The bad dog really angrily. --(Really does not work as a verb)

The bad dog howled quite. --(Quite does not work as an adverb)

Thus, this set of words does not seem to fit any of the main part of speech roles. (A similar experiment could be used to eliminate parts of speech which more obviously do not fit, such as prepositions, demonstratives, articles, conjunctions, and so on.)

(Some of the words do have several meanings where they can be other parts of speech,

The very day you eat of it you will die.

Are you really going to lick that ice cream off of the floor?

Do you like Mozart? Oh yes, quite.

but I am disregarding those other meanings.)

However, I noticed that these words do work when they modify modifiers, i.e. adjectives and adverbs, as can be seen in the following example sentences.

The very bad dog howled quite angrily.

All of the extremely smart girls will very quietly figure out a quite clever solution.

Is he really that stupid?

Just for my purposes of labelling the words in my language, I just invented a term that I think fits their role in the sentence: meta-modifier. I did notice that dictionaries list these words as adverbs.

Since adverbs can modify participles, which can be classified as adjectives,

The filthily clothed man tried to hug me.

it makes a little bit of sense. But, AFAICT, adverbs cannot modify other adverbs or non-participle adjectives, but very, extremely, really, and quite can.

So my question (finally) is, are these words really adverbs, and if not, how can these words be classified?

Answer

One problem is that the entire concept of "part of speech" is very old. How we use it in English, especially in dictionaries, goes back to the study of Latin and Greek. In this view of English grammar "adverb" is the catch-all category where everything that doesn't fit into one of the other traditional categories ends up. (The others being noun, verb, adjective, pronoun, preposition, conjunction, and interjection.)

Now there is no one, true description of any language (except perhaps constructed languages such as yours). There are merely alternative or competing descriptions which appear over time as more independent analyses of the language are undertaken. Such descriptions or analyses may be called "grammars".

Most (but not all) grammars include a concept of word class under one name or another. So one problem is that "part of speech" has two meanings. One is the specific set of eight categories from the classical languages, the other is as a synonym for word class, which is a lot looser.

So all your example words are adverbs under this older stricter view of parts-of-speech, but their qualities and quirks can be much more thoroughly investigated in newer ways. And various new ways will have various new terms for the classes they put these various words into.

Unless you are inventing a new language specifically to embrace the classical parts of speech you don't have to worry in which they belong, but if you are inventing a new language to learn more about how language works then it will be worth your time reading up on the many newer grammars and language descriptions and analyses.