choosing an adjective or an adverb
The test for picking the right modifier — check what word it's actually attached to.
Choosing between an adjective and an adverb depends on what the modifier attaches to, not on how the sentence sounds: a word describing a noun (including the subject, through a linking verb) takes the adjective form; a word describing an action verb takes the adverb form.
Examples
She sings beautifully (adverb, describes sings) vs. she feels bad (adjective, describes she through the linking verb feels).