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A subject complement is the information that follows a linking verb to describe, identify, or rename the subject of the clause. Whereas most verbs describe the action a subject performs, linking verbs describe something about the subject, which is completed by the subject complement.
A subject complement can either be a predicate noun, a predicate pronoun, or a predicative adjective.
a word or group of words, usu. functioning as an adjective or noun, that is used in the predicate following a copula and describes or is identified with the subject of the sentence, as sleepy in The travelers were sleepy.