A subject complement tells more about the subject by means of the verb. In the examples below the sentence elements are (SUBJECT + VERB + COMPLEMENT)
Mr. Johnson is a management consultant. (a predicative nominal)
She looks ill. (a predicative adjective)
An object complement tells us more about the object by means of the verb. In the examples below the sentence elements are (SUBJECT + VERB + OBJECT + COMPLEMENT). Object complements can often be removed leaving a well-formed sentence, thus the use of the term complement is slightly illogical.
We elected him chairman. (a predicative nominal)
We painted the house red. (a predicative adjective)
Adverbials, central to the meaning of a sentence, are usually adjuncts (i.e. they can be removed and a well-formed sentence remains). If, however, an adverbial is a necessary sentence element, then it is an adverbial complement. Adverbial complements often occur with a form of the copula be acting as a clause's main verb. The structure of the sentence below is (SUBJECT + VERB + ADVERBIAL COMPLEMENT)
John is in the garden.
Some grammarians refer to objects as complements.