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'Coûter cher', 'parler fort': adjectives that work as adverbs

In French you say ces billets coûtent cher (these tickets are expensive) and parle plus fort (speak louder). Here cher and fort are not describing a noun. They describe the verb, just like an adverb. They keep their basic masculine singular form and never change, even when the subject is feminine or plural. Words like bon, dur, bas, faux and lourd do the same thing in a handful of fixed verb phrases.

English speakers trip over this in two ways. First, they expect every French adverb to end in -ment, so they invent forms like chèrement where a native speaker would simply say cher. Second, they have learned that French adjectives agree, so they write elles coûtent chères. In these phrases, nothing agrees.

Word list
bonmA1 good sentir bon = to smell good chermA1 expensive coûter cher = to be expensive courtmA1 short couper court = to cut short durmA1 difficult travailler dur = to work hard fortmA1 strong frapper fort = to hit hard grandmA1 big voir grand = to think big grosmA1 big gagner gros = to win big hautmA1 tall penser tout haut = to think aloud mauvaismA1 bad sentir mauvais = to smell bad clairmA2 clear voir clair = to see clearly doubleA2 double voir double = to see double droitmA2 straight marcher droit = to walk straight finmA2 thin hacher fin = to chop finely fortmA2 loud parler fort = to speak loudly légermA2 light manger léger = to eat light lourdmA2 heavy peser lourd = to weigh a lot secmA2 dry boire sec = to drink heavily tout droitA2 straight ahead aller tout droit = to go straight on vraimA2 true dire vrai = to tell the truth basmB1 low parler bas = to speak quietly fauxmB1 false chanter faux = to sing out of tune fraismB1 cool servir frais = to serve chilled grasmB1 greasy manger gras = to eat fatty food justeB2 just chanter juste = to sing in tune netmB2 sharp s'arrêter net = to stop dead sentir mauvaisB2 to stink sentir mauvais = to smell bad troubleC1 murky voir trouble = to have blurred vision
How it works

The adjective comes straight after the verb and describes the action: how much something costs, how loudly someone speaks, how something smells. Because it works as an adverb, it stays in the masculine singular form. Compare these two sentences: ces fleurs sont chères (adjective after être, so it agrees) and ces fleurs coûtent cher (adverb after coûter, so it does not agree).

English does something similar with work hard, speak loud and smell good, which helps. But French only allows this in set combinations. You cannot turn any adjective into an adverb this way. For example, il parle lent is wrong: the correct form is il parle lentement. The safe approach is to learn each word together with its verb, as one chunk.

Ces chaussures coûtent cher.
(These shoes are expensive.)
The words
Smell and taste

With sentir, use bon and mauvais. Sentir mauvais is the everyday way to say that something stinks.

Les fleurs sentent bon, mais tes chaussettes sentent mauvais !
(The flowers smell nice, but your socks stink!)
Effort and force

Travailler dur means to work hard. Frapper fort means to hit hard; this use of fort comes from its meaning of strong.

Mes parents travaillent dur toute la semaine.
(My parents work hard all week.)
Voice

Use parler fort for loudly and parler bas for quietly. Parler haut means to speak up or speak out, and penser tout haut means to think aloud. For singing, use chanter juste (in tune) and chanter faux (out of tune).

Parle plus fort, je ne t'entends pas !
(Speak louder, I can't hear you!)
Dans la bibliothèque, on parle bas.
(In the library, people speak quietly.)
Ma sœur chante juste, mais moi, je chante faux.
(My sister sings in tune, but I sing out of tune.)
Direction and movement

Marcher droit means to walk straight, and tout droit means straight ahead. S'arrêter net means to stop dead.

Allez tout droit, puis tournez à gauche.
(Go straight on, then turn left.)
La voiture s'est arrêtée net.
(The car stopped dead.)
Seeing

Voir clair means to see clearly, both literally and in the sense of understanding a situation.

Avec mes nouvelles lunettes, je vois clair.
(With my new glasses, I can see clearly.)
Weight, food and drink

Peser lourd means to weigh a lot. Manger léger means to eat light food, and manger gras means to eat fatty food. Boire sec means to drink heavily. Gagner gros means to win big.

Cette valise pèse lourd !
(This suitcase is really heavy!)
Le soir, je préfère manger léger.
(In the evening, I prefer to eat light.)
Mon oncle a gagné gros au loto.
(My uncle won big on the lottery.)
Cutting

Couper court means to cut short, both for hair and for a conversation.

Elle a les cheveux coupés court.
(She has her hair cut short.)
Watch out

The typical mistake is adding agreement: ces robes coûtent chères is wrong. The correct form is ces robes coûtent cher. A useful test: if you could swap the word for an English adverb (expensively, loudly, hard), leave it unchanged.

The -ment forms often exist, but they usually mean something else. Fortement means strongly, as in strongly advising. Durement means harshly. Justement means exactly or precisely. So parler fort and parler fortement are not interchangeable.

Remember that the h of haut is aspirated, so there is no liaison: tout haut is pronounced without linking the two words.

Summary

In a small set of fixed phrases such as coûter cher, sentir bon, travailler dur, parler fort, chanter faux and aller tout droit, the adjective describes the verb and works as an adverb. It stays in its masculine singular form, never agrees and takes no -ment. Learn each word together with its verb, and do not create new combinations on this model.