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What heads can an adverbial phrase have? Consider the following examples:
I'll go to bed [soon]_AdvP.
I'll go to bed [in an hour]_AdvP.
I'll go to bed [when I've finished my book]_AdvP.
I'll do it [on my own]_AdvP.
It is rather easy to identify the head of the adverbial phrase in the first example, it is soon.
What is the head of the adverbial phrase in the second, third and forth example?
Is it hour, finished and own respectfully?
What do you think?
in, when and on are the syntactic heads of the respective examples. The second and the fourth example are PPs, so we can expect a preposition to appear as the head. The third example is a subordinated clause. This clause is not an adverb, but it rather an adjunct that is semantically interpreted as an adverbial.
Identifying the bracketed parts of the examples as "adverbials" is more a semantic decision that a syntactic one.
Why does there have to be a category AdvP? McCawley suggests in The Syntactic Phenomena of English that an adverb is a modifier of anything other than a noun (for which we have the special term "adjective"). So lets just use "adverb" in that way, as a term of convenience. Then for your example "I'll go to bed in an hour", if the sentence structure makes "in an hour" a modifier of "go to bed", "in an hour" is an adverb, according to the definition, but its category in the sentence structure is PP (or P-bar). If you want to reserve the term "adverb" for lexical adverbs like "immediately", we can call "in an hour" an "adverbial" instead of "adverb".
There might be a case for a category Adverb-bar, for a phrase whose head is a lexical adverb.
There is a neat characterization of "modifier" that emerges from McCawley's analyses, but which he doesn't state explicitly. A "modifier" is something that is appended to a constituent of some category to create another constituent of the same category. So "in an hour" will be a modifier of "go to bed", which let's say is a VP, if we assign the structure:
[VP [VP go to bed ] [PP in an hour ] ]
"Modifier" is another term of convenience which need not be a category that turns up in sentence structure.
Added: I see that I didn't get around to answering the question. Any head whatsoever -- that's the answer, so far as what I said above goes. I've placed no constraint on what a modifier can be (though there might be such constraints), so anything that could be the head of something could be the head of an adverbial phrase.
N', V', P', A', Adv', and 0' (zero-bar – ‘phrasal unit whose head belongs to no part of speech’)", which also distinguished N' from NP (which is not, like N', a phrase headed by an N, but rather is a different type, outside the X-Bar system – NP is the syntactic constituent type corresponding to the logical type Argument of Predicate).
Mar 2 '15 at 17:23