O mau uso de mal: corte o mal pela raiz
MAU e MAL
The two words mau and mal are different and easy to confuse. A simple rule is the following:
Use mau if you can substitute the word for the opposite bom (good).
Use mal if you can substitute the word for the opposite bem (well).
Mau (Plural: maus) is a masculine adjective:
The feminine form of mau is má (Plural: más).
uma má ideia
a bad idea
malicious gossip (lit.) bad tongues
As in em maus lençóis (in a tight spot, in a fix, in trouble, in a difficult situation):
O ex-deputado ficou em maus lençóis quando veio à tona que ele recebia propina do dono do restaurante da câmara dos deputados.
The ex-congressman got in a tight spot when it became clear that he was receiving bribes from the owner of the restaurant in the House of Representatives.
Mal can have various uses.
1. As an adverb.
Ele dormiu mal.
He slept badly.
2. To mean ‘hardly’, ‘scarcely’.
Mal o conheço.
I hardly know him.
3. As a noun.
Cortar o mal pela raiz.
To nip something in the bud. (lit.) to cut the bad at the root.
Há males que vem para o bem.
It’s an ill wind (that blows nobody any good). (lit.) there are bad things that come for the good. (A popular saying, used to say that even when something bad happens, it often has good results or brings some advantage to somebody.)
4. To mean ‘disease’.
Mal de Alzheimer
Mal de Parkinson
Mal de Chagas
Chagas’ disease (South American trypanosomiasis) – unfortunately still quite common in Brazil
Fonte: Tecla Sap