[1][2] The category is highly heterogeneous,[3]: 563 but a large number of the very typical members are derived from adjectives + the suffix -ly (e.g., actually, probably, especially, & finally) and modify any word, phrase or clause other than a noun.
Semantically, they are again highly various, denoting manner, degree, duration, frequency, domain, modality, and much more.
which you that the easier know to be an adverb, by asking the question, what, upon it, whereunto a verb, participle or adjective answers single or in sentence.
[8]: 448 When it became broadly accepted that adverbs modified more than verbs, grammarians struggled to delimit the extent of their range.
In 1784, John Hunter made the argument that many words that had sometimes been categorized as adverbs were, in fact, prepositions.
[9]: 116 This idea was taken up by later grammarians, including Jespersen (1924)[10] and The Cambridge grammar of the English language (2002).
[11] Although most modern linguists accept the idea that lexical categories, such as adverb, cannot be defined semantically, Langacker argues that they can.
Aarts summarizes this: "Verbs designate processes, whereas adjectives and adverbs are said to designate atemporal relations"[12][13]: 377 In the general case, adverbs do not function as attributive modifiers in a noun phrase, where adjectives typically do.
[a] There are cases, though, in which an adverb may be a non-attributive modifier in a noun phrase (NP), as in the situation recently in Spain.
[4]: 123 Some adverbs also share pronunciation and spelling with adjectives (e.g., very),[14] which makes such tests inconclusive; you can't rule out the possibility that a word is an adverb simply because a word spelled that way can be a modifier in an NP.
Adverb phrases rarely license complements as dependents, but it is possible, as in independently of the others.
For example, the adverbs straight and right characteristically function as modifier in PPs but not in AdvP (e.g., right up the tree but not *right vertically).
[3]: 603 Similarly, very characteristically functions as modifier in AdjPs and AdvP but not in VPs (e.g., very happy & very happily, but not *very enjoyed).
[4]: 159 A comprehensive grammar of the English language gives the following (non-exhaustive) list of semantic roles for "adverbials":[18] space, time, process, respect, contingency, modality, and degree.