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'Falar alto', 'custar caro': adjectives that work as adverbs

In Portuguese, a handful of everyday adjectives can describe how something is done, with no change to their form. People falam alto (speak loudly), things custam caro (cost a lot), you respira fundo (breathe deeply) and corre rápido (run fast). In this role the word works as an adverb, and it always stays in the masculine singular, whoever or whatever the sentence is about.

English speakers trip over this in two ways. First, Portuguese adjectives normally agree with nouns, so it feels natural to say elas falam altas. That is wrong. Second, English learners are taught that adverbs end in -ly, which leads them to reach for -mente forms that sound odd or don't exist. English actually has a few similar cases (work hard, go straight, aim high), and Portuguese simply has many more.

Word list
baratomA1 cheap comer barato = eat cheaply bonitomA1 beautiful cantar bonito = sing beautifully (colloquial) caromA1 expensive custar caro = cost a lot difícilA1 difficult falar difícil = use fancy words (elas falam difícil) fácilA1 easy ganhar fácil = win easily (colloquial) forteA1 strong bater forte = hit hard lindomA1 beautiful cantar lindo = sing beautifully (colloquial) rápidomA1 fast correr rápido = run fast altomA2 loud falar alto = speak loudly claromA2 clear falar claro = speak plainly diferenteA2 different pensar diferente = think differently (elas pensam diferente) erradomA2 incorrect escrever errado = spell wrong (colloquial) estranhomA2 strange agir estranho = act strangely (colloquial) feiomA2 ugly fazer feio = perform badly (colloquial) finomA2 thin falar fino = speak in a high-pitched voice (opposite of falar grosso) fundomA2 deep respirar fundo = breathe deeply leveA2 light pegar leve = go easy (colloquial) limpomA2 clean jogar limpo = play fair pesadomA2 heavy pegar pesado = go too far, be harsh (colloquial) retomA2 straight seguir reto = go straight on sujomA2 dirty jogar sujo = play dirty baixomB1 low falar baixo = speak quietly diretomB1 direct ir direto = go straight (to) falsomB1 false soar falso = ring false; jurar falso = swear falsely grossomB1 thick falar grosso = talk tough sériomB1 serious falar sério = be serious, not joking
How it works

The pattern is simple: verb + adjective in the masculine singular. The adjective describes the action, not a person or a thing, so it has nothing to agree with. Compare it with an adjective that really does describe the subject. That one agrees as usual.

As crianças chegaram cansadas e falaram alto.
(The children arrived tired and spoke loudly.)

Cansadas tells you about the children, so it is feminine plural. Alto tells you about the speaking, so it stays alto. A quick test: if you can replace the word with assim (like this), it is adverbial and does not change. Words like muito, mais and tão can still go in front: muito alto, mais baixo, tão caro.

The words
Voice and way of speaking

Alto and baixo describe volume. Falar claro means to speak plainly or frankly, falar sério means to be serious, not joking, and falar grosso means to talk tough or lay down the law.

Por favor, falem baixo na biblioteca.
(Please speak quietly in the library.)
Vou falar claro: não gosto deste plano.
(I'll be frank: I don't like this plan.)
Ela não estava brincando, estava falando sério.
(She wasn't joking, she was being serious.)
O chefe falou grosso com a equipe.
(The boss talked tough with the team.)
Prices

Caro and barato go with verbs like custar, pagar, comer and sair.

Essas botas custaram caro.
(Those boots cost a lot.)
Nesta cidade come-se barato.
(You can eat cheaply in this city.)
Speed, depth and force
As minhas irmãs correm muito rápido.
(My sisters run very fast.)
Respire fundo e conte até dez.
(Take a deep breath and count to ten.)
Choveu forte a noite toda.
(It rained hard all night.)
Direction and fair play

Reto and direto describe the path you take. Limpo and sujo describe how fairly someone plays.

Siga reto até a praça e depois vá direto ao hotel.
(Go straight on to the square and then go straight to the hotel.)
Eles jogaram sujo, mas nós jogamos limpo.
(They played dirty, but we played fair.)
Colloquial, mostly Brazilian

Bonito, feio, errado and fácil are used this way mainly in relaxed Brazilian speech. Fazer feio means to perform badly or embarrass yourself, and ganhar fácil means to win easily.

Elas cantaram bonito, mas a banda fez feio.
(They sang beautifully, but the band did badly.)
O time ganhou fácil, mas vocês escreveram errado o nome do jogador.
(The team won easily, but you spelled the player's name wrong.)
Watch out
No agreement, ever

Say as casas custam caro, not custam caras. Compare a casa é cara, where cara describes the house and agrees as normal.

The -mente trap

Some -mente forms exist but mean something else or sound stiff. Altamente means highly, as in altamente recomendado, not loudly. Nobody says caramente or fundamente. Rapidamente and claramente are fine words, but correr rápido and falar claro are the everyday choices, and falar claro also suggests frankness.

Portugal vs. Brazil

In Portugal you will often hear falar a sério rather than falar sério. The colloquial uses of bonito, feio, errado and fácil sound more natural in Brazil.

Fixed pairings

These words don't work with just any verb. Learn them as chunks: respirar fundo, bater forte, jogar limpo. Others like trabalhar duro (work hard), segurar firme (hold tight) and mirar alto (aim high) follow the same pattern.

Summary

When an adjective comes right after a verb and describes how the action is done (falar alto, custar caro, respirar fundo, correr rápido), it is working as an adverb. Keep it masculine singular, whoever the subject is, and don't add -mente. Learn these as fixed verb + word chunks.