Mal vs Mau:
"Mau" describes the character of a bad person or thing, while Mal mostly describes doing something badly or feeling bad.
For example: "O meu chefe é mau." (my boss is a bad person), "O meu chefe canta mal." (my boss sings badly)
Word Variations
Má
Maus
Más
Examples
Não seja mau com seu irmão. Devolva o notebook dele.
Don't be mean to your brother. Return his notebook.