What really makes adverbs different from adjectives?

Question

I just tried to answer a question that amounted to knowing whether adverbs can be inflected. Then, doing a bit of search for examples, I came up with the impression that, in many cases, I could not tell adverbs from adjectives from looking at uses (though I wonder whether that is not more specific of US English).

For example the adjective kind can be derived into the adverb kindly that is also used as an adjective (probably a million occurrences on the web), and is indeed presented as both by Google.

It seems however that comparative is different as, for some reason, adverbs are supposed to form the comparative only with more, while short adjective can do it by inflection such as "kindlier". (minor question: Is that an absolute rule?)

My question really is more about the distinction between adverbs and adjectives as distinct parts of speech.

As John Lawler puts it:

Adverbs have long been called a ‘wastebasket’ category in syntax. Their definition is very general: adverbs are distinguished from adjectives, which modify nouns, by saying that ‘adverbs modify verbs, adjectives, or other adverbs’; to this one can add that they may also modify phrases and clauses as a whole.

I am not sure I understand why nouns should have their special part of speech as modifier, while all others get a common one. I am even less sure when I find that very many words can be used as both, at least in English. It does not seem to be the same in some/many other languages, but there is no reason that parts of speech should always be the same, or is there?

I would tend to paraphrase John Lawler by saying, with apologies for the abuse:

"Noun adverb" is not a type of adverb.
It's a type of adverbial construction, or usage;
one of the things some adverbs can do.

Sorry if the question seems naive, or is due to my limited vision of the language. I would be interested in knowing the current wisdom on this, and why these remarks may or may not make sense.

Answer 1

If we step off linguistic terminology to some philosophy, everything becomes more straightforward.

Adjectives define properties of "things";
Adverbs define properties of "relations".

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TL;DR

Human logic operates with two fundamental categories, "things" and "relations".

Things are linguistically represented with nouns, pronouns, and noun-like entities like Gerund and Verbal Noun in English (other languages have similar constructs as well).

Relations can be of two fundamental types: (1) relation between two "things" in a sentence or a "thing" against itself, or time, location, etc. (verbs and verb-like constructs) and (2) relation between a "thing" and similar "things" out of the context.

Both adverbs and adjectives belong to the 2nd category:

Adjective. Think of what is the actual purpose of "green" in "a green apple"? Here, "green" defines a certain property (attribute) by which this apple is different to other apples out of the context.

Similarly, Adverb. "Fast" in "to run fast" denotes a relation of a certain "way to run" against other "ways to run".
Obviously, Adverb can be "attached" to an adjective: "a very green apple" denotes some property (attribute) of "greenness" against other "greennesses".
...or to another adverb: "to run very fast".

Now, we can answer your question in a very simple manner:

why nouns should have their special part of speech as modifier, while all others get a common one.

It's not because of nouns. It's because of "things" and "relations".

Adjectives define attributes of things (nouns, pronouns, etc.)
Adverbs define attributes of relations, where relations can be actions (verbs) or other attributes (adjectives or adverbs).

P.S. Yet again, sorry for a rather philosophic answer than a linguistic one. I'm sure I'm not the first to suggest this, but I'm sincerely not aware about any academic researches over this matter.

Answer 2

Their definition is very general: adverbs are distinguished from adjectives, which modify nouns, by saying that ‘adverbs modify verbs, adjectives, or other adverbs’

You have (mostly) answered yourself by giving that definition. Adjectives describe nouns, and the adverbs describe other parts of speech. In English, the same word function often as many part of speeches, depending on the position in the sentence, so that distinction is not so obvious (distinction by slow -> go slowly, but no distinction fast -> go fast)

I am not sure I understand why nouns should have their special part of speech as modifier

This is because nouns in numerous languages has a special property: gender or class. If the adjectives must agree in gender/class with the noun, you have there very clear distinction:

• adjectives are changing part of speech (declension)
• adverbs are unchanging part of speech

In the languages that has free word order the distinctive endings of the adverb allow also to determine, which words is described by them (noun or verb).

But the significance of the distinction between adjective and adverb depends on the language. In Polish it's quite fundamental, but I suppose, in Chinese it's used mostly by linguists.