A subject complement gives us more information about the subject. It usually comes after linking verbs and sense verbs (including be, seem, smell, taste), and after change of state verbs (including go, get, become).
Subject complements: parts of speech
Subject complements can be adjective phrases, noun phrases, adverb phrases or prepositional phrases:
That rice tastes quite sweet. (subject + adjective phrase)
It seems a long time since this morning. (subject + noun phrase)
Where are you?
I’mupstairs. (subject + adverb phrase)
It still smells of paint in here. (subject + prepositional phrase)
Subject complements are not the same as objects.
He marrieda famous writer.
a famous writer is a different person = the object
He becamea famous writer.
become is a linking verb; a famous writer describes the subject = the same person as he
Complements and adjuncts are different. A complement is necessary in order to complete the meaning. An adjunct is not necessary, and adds extra information.