Chapter 6-1-2: Using Adjectives with Linking Verbs
In the same vein, remember that adjectives modify nouns and pronouns. Do not mistakenly use an adverb to modify these parts of speech.
For example, after a linking verb you may be tempted to use an adverb instead of an adjective. You will recall that the linking verb is a special kind of verb because it links its subject to a subject complement. A subject complement can be either a noun (renaming the subject) or a modifier (describing the subject). When it is a modifier it must be an adjective because it describes the subject (always a noun or pronoun). It does not modify the linking verb itself and should therefore not be an adverb:
[WRONG] We felt badly about having caused the accident
[RIGHT] We felt bad about having caused the accident.