Mau vs. ruim
In Portuguese, mau and ruim both act as adjectives meaning bad, but their usage differs significantly by region. Mau is the direct grammatical opposite of bom (good). While usually interchangeable in definition, Brazil tends to favor ruim for everyday descriptions of quality, whereas Portugal uses mau as the standard word for almost all negative contexts.
Mau
A1This word is the direct antonym of bom (good). It refers to poor quality, moral evilness, or something harmful. In European Portuguese, mau is the default word for bad in all situations. In Brazilian Portuguese, mau is used more often to describe character flaws (evil) or in specific set phrases.
Ele é um mau exemplo para as crianças.
(He is a bad example for the children.)
Acordei de mau humor esta manhã.
(I woke up in a bad mood this morning.)
O lobo mau soprou a casa.
(The big bad wolf blew the house down.)
O tempo está muito mau em Lisboa hoje.
(The weather is very bad in Lisbon today.)
Ruim
A2This adjective describes something unpleasant, defective, or of poor quality. In Brazil, ruim is the most common spoken choice to say a movie, food, or situation is bad. In Portugal, ruim is used much less frequently; when used, it often sounds archaic or implies a specific nastiness or wickedness.
Esta comida está com um cheiro ruim.
(This food has a bad smell.)
O filme era tão ruim que saí do cinema.
(The movie was so bad that I left the theater.)
A minha internet está muito ruim hoje.
(My internet is very bad today.)
Ficar doente nas férias é muito ruim.
(Getting sick on vacation is very bad.)
Summary
To sound natural, use ruim in Brazil for everyday complaints about quality (like bad food or bad weather), and keep mau for moral contexts (evil) or fixed expressions. In Portugal, you can safely use mau for everything that is the opposite of good.