Level of Distractibility (i.e., the degree to which readers are distracted from meaning by mistaken usage): Who cares? = *; some people will be distracted = **; many people will be distracted = ***.
Question: When shoud I use "bad" and "badly"?
Use "bad" after the verbs "be," "appear," "look," "feel," "seem," "smell" or "taste." Example: "I feel bad." I could give a lengthy grammatical explanation of why these verbs, called "linking verbs," are different from action verbs, but the explanation would be complicated and not worth the time and effort.
Note: If the correct use of "bad" after these verbs sounds "bad," write around it. Example: "He appears bad." "He appears to be seriously hurt." You will probably use more precise word choice. Example: "It sounds bad." "His choice of words is awkward."
Use "badly" after an action verb: "He has been hurt badly. "
The bottom line? NEVER, NEVER say or write, "I feel badly."
As the New York Times Manual of Style and Usage says, "Someone who smells bad should bathe; someone who smells badly should see a doctor."
Rating of Distractibility: ***