What is the difference between mal and mauvais? Let me tell you...

1. Mal ou mauvais ?
Mauvais is (usually) an adjective (modifies a noun)
It means ‘bad’, or (depending on the context) it can mean ‘incorrect’/’wrong’,
‘mean’/’unkind’, etc.
Mal is (usually) an adverb (modifies a verb / “doing word”)
It means ‘poorly’, ‘badly’ or ‘improperly’.
It can also be used in front of a past participle to give a negative meaning.

2. Mal ou mauvais ?
Mauvais
Une mauvaise idée - a bad idea
Mauvaise chance - bad luck
Il fait mauvais - the weather is bad (lit. it ‘makes’ bad)
Mal
Un mal-entendu - a misunderstanding (lit. a bad hearing)
J’ai mal fait - I did it wrong
Elle voit mal - she sees poorly/ badly