bon vs bien
Summary
bon is an adjective ("good") that describes a noun and agrees with it in gender and number, while bien is mainly an adverb ("well") that modifies a verb, adjective or adverb and never changes form.
Explanation
The core split is grammatical: bon is an adjective, bien is an adverb. Bon means "good" and qualifies a noun, agreeing with it in gender and number: un bon repas, une bonne idée, de bons amis, de bonnes notes. Because it is an adjective, it has four written forms (bon, bonne, bons, bonnes). Bien means "well" and modifies a verb, an adjective, or another adverb, and it is invariable — one single form whatever the subject: elle chante bien, ils travaillent bien, c'est très bien.
After être or another linking verb, the choice depends on what you are really describing. Bon judges the intrinsic quality of a thing, especially taste: ce gâteau est bon, la soupe est bonne. Bien comments on a state, a situation, or moral/aesthetic worth rather than flavour: tout va bien, il est bien ici, ce film est bien (it's good/worthwhile). So ce plat est bon praises how it tastes, while ce restaurant est bien praises the place overall.
Two frozen expressions capture the everyday contrast. C'est bon is about sensation and completion — food that tastes good, or "that's fine / that's enough" (c'est bon, j'ai compris). C'est bien is about quality, behaviour or approval — "that's good / well done / that's the right thing" (c'est bien d'aider les autres). Mixing them flips the meaning, so they are worth memorising as fixed pairs.
Note that each word has a secondary use that does not break the rule. Bien can act like an intensifier or quantity word meaning "quite/really/many": bien sûr, bien content, bien des gens. Bon can serve as a discourse marker (Bon, on y va !) or a noun (un bon de réduction, a voucher). These extras are common, but the agreement test still tells you which base word you need.
Decision rule: if the word describes a noun and could agree (good + thing), use bon and make it agree; if it tells you how an action is done or comments on a manner/state (well), use bien and leave it unchanged. Quick test for food vs performance: praising taste → c'est bon; praising how something is done or its overall quality → c'est bien / il le fait bien.
Examples
C'est un bon restaurant. — It's a good restaurant. (Bon: adjective describing the noun restaurant (masculine singular).)
Elle parle très bien français. — She speaks French very well. (Bien: adverb describing how she speaks; invariable.)
Ce gâteau est vraiment bon. — This cake is really good. (After être, bon judges taste; it agrees (here masculine singular).)
C'est bien de dire merci. — It's good to say thank you. (C'est bien: approval of behaviour, not taste — invariable.)
Ce sont de bonnes idées. — These are good ideas. (Bon agrees: feminine plural bonnes with idées.)
Tout s'est bien passé. — Everything went well. (Bien modifies the verb se passer; no agreement.)
Common mistakes
Elle parle bon français.
Elle parle bien français.
To say how an action is performed, French needs the adverb bien (invariable), not the adjective bon. « Elle parle bien français » = she speaks French well. Bon is an adjective and qualifies a noun, agreeing with it (un bon film, une bonne idée). The decision rule: modifying a verb (how?) -> bien; describing a noun (what kind?) -> bon.
Prerequisites
Related rules
adjective-adverbconfusablesagreement