@David2019, @danieljgoldstein is correct 
Mal/bem (badly/well) → Adverbs
Mau/bom (bad/good) → Adjectives
But @mvrudloff is also partly correct when noting that “mal” (and “bem”) can be used as a noun. Some examples of all these:
Use of ‘mal’ as a noun
- O mal não vencerá! (Evil shall not win!)
- Ele sofre de um mal severo. (He suffers from a severe illness)
Use of ‘mal’ as an adverb
- Eu canto mal. (I sing badly)
- Nós fomos mal recebidos. (We were badly welcomed).
Use of ‘mau’ (adjective)
- Tu és uma má pessoa. (You’re a bad person)
- Este é um mau sinal. (This is a bad sign)