Bad is an adjective that everyone generally knows. It is the opposite of good, poor quality or not well.
The adverb form of bad is badly.
Confusion comes when one needs to know whether or not to use the adjective form or the adverb form. Action verbs, which describe an activity or movement, need adverbs to modify how the action is being done. Linking verbs, which connect a state of being verb like to be to its state, require adjectives to modify them. Think of it as either modifying the verb or the subject, since state of being verbs are directly related to the subject.
To further complicate things, some verbs can be a linking verb or an action verb. Most of these have to do with the five senses. You can actively feel sandpaper or you can feel happy. You can look closely at something or look pretty in a dress. Lastly, you can smell badly because you have a cold or you can smell bad because you haven’t taken a shower.
When it doubt, try to replace the verb with was and see if it still sounds right. I am badly doesn’t make sense. I am bad does.
However, the last complication makes all of this moot. An alternative adverb form for bad is bad. Caution should be used because this is informal and, if we had to guess, based solely on erroneous usage of bad. It is best to take the time and make sure you are using the right form.