Mau vs. mal
The Portuguese words mal and mau are a common source of confusion for learners as they sound almost identical and can both be translated as bad. The key difference lies in their grammatical function: mau is an adjective that describes nouns, while mal is primarily an adverb that describes verbs.
Mau
A1Mau is an adjective, which means it is used to describe a noun (a person, place, or thing). It must agree in gender and number with the noun it modifies, having the forms mau (masculine singular), má (feminine singular), maus (masculine plural), and más (feminine plural). Its direct opposite is bom (good).
Ele é um homem mau.
(He is a bad man.)
O lobo mau é um personagem famoso.
(The big bad wolf is a famous character.)
Tive um mau pressentimento sobre a viagem.
(I had a bad feeling about the trip.)
Ela tomou uma má decisão.
(She made a bad decision.)
Não ande em más companhias.
(Don't hang around with bad company.)
Eles têm maus hábitos alimentares.
(They have bad eating habits.)
O cheiro no quarto estava muito mau.
(The smell in the room was very bad.)
Mal
A1Mal is most commonly an adverb, meaning it modifies a verb, an adjective, or another adverb to describe how an action is performed. As an adverb, it is invariable (it does not change form). Its direct opposite is bem (well). Mal can also function as a noun meaning evil or illness, or as a conjunction meaning as soon as.
Eu dormi mal esta noite.
(I slept badly last night.)
O aluno respondeu mal à pergunta do professor.
(The student answered the teacher's question poorly.)
A comida foi mal preparada.
(The food was poorly prepared.)
Não me sinto bem, estou com um mal-estar.
(I don't feel well, I have a feeling of unease/sickness.)
O bem sempre vence o mal.
(Good always triumphs over evil.)
Mal cheguei em casa, o telefone tocou.
(As soon as I got home, the phone rang.)
Ela sofre de um mal desconhecido.
(She suffers from an unknown illness.)
Summary
In short, the easiest way to decide between mau and mal is to think of their opposites. If you can replace the word with bom (good), then the correct word is mau. If you can replace it with bem (well), then the correct word is mal. Remember: mau is an adjective that describes things (bad man / homem mau), while mal is an adverb that describes actions (slept badly / dormiu mal).