My mother is now back with us.
Does "back" in the sentence function as an adverb? Can adverb phrase function as subject complement?
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My mother is now back with us.
The phrase back with us is a preposition phrase. It is headed by the preposition back. The Original Poster asks if back can be considered a subject complement. It does indeed describe the subject, my mother. Because it tells us the location of my mother, we call it a locative complement.
We often use preposition phrases as locative complements:
We can show that back is a preposition and not an adverb. The Original Poster asks whether adverbs can function as Subject Complement. The answer is that adverbs can NOT normally function as complements of the verb BE - but prepositions and preposition phrases can:
Adverbs as complements of BE
Preposition Phrases as complements of BE
Also, adverbs can't usually modify noun phrases. Prepositions and preposition phrases can:
Adverbs modifying nouns
Preposition Phrases modifying nouns
In addition, adverbs can usually be modified by the adverb very, prepositions usually cannot:
Adverbs modified by very
Prepositions modified by very
Lastly, most prepositions and preposition phrases can be modified by the special adverbs straight and right. Adverbs cannot usually be modified by straight or right:
Adverbs modified by straight or right
Prepositions modified by straight or right
The Original Poster's question
In the Original Poster's example the preposition back has another preposition phrase as its complement. The complement of the preposition back is with us. This second preposition phrase has the preposition with as its head. The complement of with is the pronoun us. The whole preposition phrase back with us functions as the complement of the verb BE. It is a locative complement telling us the location of the subject, My mother.
Adverbs, on the other hand, do not usually function as complements of the verb BE. We do not usually find them, therefore, as subject complements.
Hope this is helpful!
In your sentence, 'back' is being used as an adverb of place. I think the issue here is that 'Mother' (a person) cannot equal 'back' (a location). You can't have a 'back mother.' That is nonsense.
But an adverbial phrase CAN be a subject complement. I'll throw this out:
You have to be smarter than the horse you are riding.
In this example, 'smarter' is a complement to the subject, 'you.' The two are equivalent. In this case, 'You' (a person) can equal 'smarter' (an adjective). A big man. A yellow horse. These are possible. A 'smarter you' is also completely possible.
'Smarter than the horse' also happens to be an adverbial phrase of comparison. 'Than' is a correlative conjunction, connecting the grammatically equal 'you' and 'horse.' So the phrase 'smarter than the horse' becomes -- as a whole -- a subject complement. And it is an adverbial phrase. Behold, the two are one.
But I can't leave something unfinished. So:
'You are riding' has an elided 'that,' making it a non-defining relative clause.
I believe 'to be' is an infinitive direct object to 'have,' though some might consider 'have to be' a complex verb. For me, it is easier to think of infinitives as a discrete units functioning in specific ways instead of tacking them to whatever happens to be close by.
Yes, an adverb can also function as a subject complement in some cases. The word "back" in the sentence "My mother is now back with us" has been used as an adverb.
Cambridge English Grammar Today states:
"Subject complements can be adjective phrases, noun phrases, adverb phrases or prepositional phrases. I am upstairs (subject + adverb)".
My mother is now back with us
This is not a link verb + subject complement construction.
(Note: I use the term complement loosely - as any word phrase that adds information to another word phrase. They do not need to be required to be called a complement.)
Sometimes BE functions as a state verb, meaning 'to exist'. This is almost always the case when BE takes a prepositional phrase as a verb complement. Noun phrases and adjective phrases after BE almost always select BE as a linking verb. Further, prepositional phrases as main verb complements almost always function adverbially (to say where, when, why, how the action or state occur).
Also, back with us is a not an adverb or an adverb phrase, it's a compound adverbial (adverb phrase + prepositional phrase). with us is an adverb complement - it is adding information to the adverb back. The compound adverbial locates where the state of 'existence' or 'being' takes place (back with us) but that doesn't make it an adverb.
It's actually the relation between the subject and the main verb that locates the state in the subject. The subjects of state verbs generally take the semantic role of possessor of that state.
So, in terms of the OP question, the only adverbs in your sentence are now and back. now is superfluous (the verb is already present tense), and it has no direct relation to the subject - it is both syntactically and semantically a verb complement.
Back is an adverb phrase as a verb complement meaning roughly (to return). With us is an adverb complement adding information to back. Together they form a compound adverbial.
My mother is back with us. S|V|A[AVP + avc(pp):back] + a(avp)
Does "back" in the sentence function as an adverb?
Yes, it is an adverb. But it is not a subject complement. It is saying where the verb occurs (back with us).
Can adverb phrase function as subject complement?
No, I don't think so. Maybe never, but possibly with non-finite clauses.
Adverb indicates a word function (class/PoS).
Adverb phrase indicates a word phrase type (one with an adverb at its head)
Adverbial indicates a word phrase or group of syntactically linked word phrases that function as verb complements to the main verb to say where, when, why or how the verb occurs.