This is “Predicate Adjectives”, section 1.14 (from appendix 1) from the book Writers' Handbook (v. 1.0).
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Since linking verbs express a state of being instead of an action, adjectives are used after them instead of adverbs. An adjective that follows a linking verb is referred to as a predicate adjective. Be careful not to use an adverb simply because of the proximity to the verb.
Correct (adjective follows linking verb): Kelly is selfish.
Incorrect (adverb follows linking verb): Kelly is selfishly.
Correct (adjective follows linking verb): Beth seems eager.
Incorrect (adverb follows linking verb): Beth seems eagerly.