adjective / adverb after linking verbs
Whether the word after linking verbs such as be, seem, feel, look, smell, sound, taste, and act describes the subject or the action.
After a linking verb, use an adjective when you describe the subject: She looks calm. The soup tastes salty. I feel bad. Use an adverb when you describe how the action is done: She looked calmly around the room. He acted carefully. Some of those verbs (such as look and act) are also ordinary verbs; when they name an action, they take an adverb.
Examples
UseI feel bad about the mistake. (adjective after a linking verb)
AvoidI feel badly about the mistake. (linking verb; needs the adjective bad)
UseThe board seemed unanimous. (when seem is a linking verb, it takes an adjective)
UseHe looked carefully at the contract. (when look is an ordinary verb, it takes an adverb)
Good after feel or look is an adjective describing the subject (She looks good; I feel good). Well is the adverb of manner and also an adjective of health (I feel well). See also bad / badly.