Bad vs. Badly
The word bad is an adjective used to modify nouns and pronouns.
Example: She was in a bad accident.
Adverbs often end in ly. The word badly is an adverb that answers how about the verb.
Example: She was hurt badly in the accident.
The confusion comes with the sense verbs: taste, look, smell, and feel.
When we use these verbs actively, we should follow them with adverbs.
When we use these verbs descriptively, we should follow them with adjectives.
Example: I feel bad about having said that.
I am not feeling with fingers in the above example; I am describing my state of mind, so the adjective is used (no ly).
Example: She feels badly since her fingers were burned.
She feels with her fingers here so the adverb (ly form) is used.
You can use this same rule about sense verbs with adjectives and adverbs other than bad and badly.
Example: The mask over his face made him look suspicious to the police.
He did not look with eyes. Look describes his appearance so the adjective is needed.
Example: She looked suspiciously at the $100 bill.
She looked with eyes so the adverb is needed.
Example: She looked good for someone who never exercised.
She didn’t look with eyes. Good is describing her appearance so the adjective is needed.
Example: He smelled well for someone with a cold.
He is actively smelling with his nose so the adverb is needed.
Rule: Well is used when referring to health.
Example: He doesn’t feel well enough today to come to work.