French adjectives used as adverbs

By now you know that French adjectives normally agree with the nouns they modify, but you've probably seen a few cases where they don't. There are a number of French adjectives which are often used as adverbs (that is, they modify verbs rather than nouns), and when used in this way, "adverbial adjectives" are invariable. These adjectives tend to be short and very common - here are the main ones.

bas 
Elle parle bas - She speaks quietly

bon
Ça sent bon - That smells good

cher
Cela coûte cher - That's expensive

clair
Parlons clair - Let's be frank

court
Ils ont coupé court aux rumeurs - They put a stop to the rumors

creux
La boîte sonne creux - The box sounds empty

doux
Il faut filer doux - You have to toe the line

droit
Ils marchent droit - They are walking straight

dru 
La pluie tombe dru - The rain falls heavily

dur 
Elle travaille dur - She works hard

faux
Ils chantent faux - They sing out of tune

ferme  
Elle boit ferme - She drinks heavily

fin 
Elle écrit très fin - She writes very small

fort 
Ils parlent fort - They talk loudly

franc 
À vous parler franc... - To be frank with you...

grand
Elle voit toujours grand - She always thinks big

gras
Elle mange gras - She eats fatty foods

gros
Elle écrit gros - She writes big

haut 
Ils ont mis les livres plus haut - They put the books higher

jeune 
Elle se coiffe jeune - She has a young person's hairstyle

juste 
Les calculs tombent juste - The calculations work out exactly

lourd 
Elle ne pèse pas lourd pour eux - She's no match for them

mauvais 
Ça sent mauvais - That smells bad

menu 
la viande hâchée menu - finely chopped meat

net
Elle s'est arrêtée net - She stopped dead

profond
Elle doit creuser très profond - She has to dig very deep

sec
Ma voiture démarre sec - My car starts with a jolt

vieux
Ces vêtements font vieux - These clothes make (you, him) look old