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.