bad, badly

  edgood  —  Grammar Tips
Bad ordinarily acts as an adjective, badly as an adverb. Bad normally describes how things are while badly describes how things might act or appear

Many people make mistakes with the linking verb feel. A linking verb links the subject of a sentence to an adjective (bad), not to an adverb (badly). Thus:
I feel bad about that.
Example: The outcome was bad because we acted badly.