How do you form an adverb of manner in French?

Start from the feminine form of the adjective and add -ment. Lent (slow) gives the feminine lente, then lentement (slowly). Heureux gives heureuse, then heureusement (happily, luckily). Doux gives douce, then doucement (gently). Going via the feminine is the key step - it's why so many adverbs carry the feminine consonant before -ment. There are a few exceptions: adjectives already ending in a vowel use the masculine (vrai gives vraiment), and the -ant / -ent adjectives follow a separate -amment / -emment pattern.

Why is it 'vraiment' and not 'vraiement'?

Because adjectives that end in a vowel in the masculine attach -ment directly to that masculine form, with no feminine -e in between. Vrai (true) ends in a vowel, so it's vrai + ment = vraiment (truly), not vraie + ment. The same goes for poli giving poliment (politely), absolu giving absolument (absolutely), and joli giving joliment (prettily). The feminine-form rule only kicks in when the masculine ends in a consonant, where you need the feminine to expose the sound that -ment attaches to.

What are the irregular French adverbs that don't use -ment?

The three big everyday ones are suppletive - they come from a different root entirely. Bien (well) is the adverb of bon (good), mirroring English good versus well exactly. Mal (badly) is the adverb of mauvais (bad). Vite (quickly, fast) is just its own word with no -ment form at all - rapidement exists as a -ment alternative, but vite is the common one. These three turn up constantly, so they're worth learning as fixed items rather than trying to derive them.

French Adverbs of Manner

An adverb of manner says how something is done: slowly, happily, gently. English mostly adds -ly; French mostly adds -ment. The trick is that French builds it off the feminine form of the adjective, not the masculine.

The core rule: feminine adjective + -ment

Take the feminine form of the adjective, then add -ment.

Adjective (masc)	Feminine	Adverb	English
lent	lente	lentement	slowly
heureux	heureuse	heureusement	happily, luckily
doux	douce	doucement	gently
franc	franche	franchement	frankly
premier	premiere	premierement	firstly
naturel	naturelle	naturellement	naturally

lent -> lente -> lentement (slowly)
heureux -> heureuse -> heureusement (happily / luckily)
doux -> douce -> doucement (gently)

Going via the feminine is what exposes the consonant that -ment latches onto. That's why lentement has the feminine -te and doucement has the feminine -ce. If you build it off the masculine you get the wrong form. The feminine endings themselves are on the adjective agreement page.

Adjectives ending in a vowel: use the masculine

If the masculine adjective already ends in a vowel, attach -ment straight to that masculine form. No feminine -e in between.

vrai -> vraiment (truly)
poli -> poliment (politely)
absolu -> absolument (absolutely)
joli -> joliment (prettily)
passionne -> passionnement (passionately)

So it's vraiment, not "vraiement". The vowel-ending masculine is already pronounceable, so French doesn't bother routing through the feminine.

The -ant / -ent adjectives: -amment / -emment

Adjectives ending in -ant drop it for -amment; adjectives ending in -ent drop it for -emment.

Adjective	Adverb	English
constant	constamment	constantly
courant	couramment	fluently
evident	evidemment	obviously
recent	recemment	recently
frequent	frequemment	frequently
patient	patiemment	patiently

constant -> constamment (constantly)
evident -> evidemment (obviously)
recent -> recemment (recently)

Pronunciation note: -amment and -emment sound identical - both are pronounced "a-ment". So constamment and evidemment rhyme exactly, even though one is spelled with -a- and the other with -e-. The spelling follows the adjective ending; the sound doesn't distinguish them. (The common exception is lentement, which comes from lent but keeps the regular feminine-plus-ment route because lent's feminine is lente, not an -ent adverb form.)

The ones that take an accent

A small set adds an acute accent (an e accent aigu) before -ment, turning the linking vowel into -ement.

precis -> precisement (precisely)
enorme -> enormement (enormously)
profond -> profondement (deeply)
commun -> communement (commonly)
aveugle -> aveuglement (blindly)

These have to be learned as they come, but the accented -ement ending is the shared signal.

The big irregulars: bien, mal, vite

Three of the most common manner adverbs don't use -ment at all. They're suppletive - a different word entirely.

Adjective	Adverb	English
bon (good)	bien	well
mauvais (bad)	mal	badly
(rapide)	vite	quickly, fast

bon -> bien (good -> well) - this mirrors English exactly: good becomes well, bon becomes bien.
mauvais -> mal (bad -> badly).
vite (quickly, fast) is just its own word with no -ment form. rapidement exists as a regular alternative, but vite is the everyday one.

These three turn up in nearly every conversation (je vais bien, il chante mal, viens vite), so learn them as fixed items rather than deriving them. The bon / bien split is the same one behind the meilleur / mieux pair on the comparatives page.

Position of the adverb

Keep it simple. In a simple tense, the adverb of manner goes after the conjugated verb:

Il parle lentement. (He speaks slowly.)
Elle travaille bien. (She works well.)

In a compound tense (passe compose and friends), short common adverbs go between the auxiliary and the past participle, while longer ones tend to come after the past participle:

Il a bien mange. (He ate well.) - short adverb, before the participle.
Elle a vite compris. (She understood quickly.) - short adverb, before the participle.
Il a repondu poliment. (He answered politely.) - longer -ment adverb, after the participle.
Elle a parle franchement. (She spoke frankly.) - longer -ment adverb, after the participle.

The rough rule: the shorter and more common the adverb (bien, mal, vite, deja, trop), the more likely it slips inside; the longer -ment adverbs sit after the participle.

Worked examples

Elle conduit prudemment. (She drives carefully.)
Heureusement, il n'a pas plu. (Luckily, it didn't rain.)
Parle plus doucement, s'il te plait. (Speak more gently, please.)
Il parle couramment l'espagnol. (He speaks Spanish fluently.)
Evidemment, elle avait raison. (Obviously, she was right.)
Je me sens vraiment mieux aujourd'hui. (I really feel better today.)
Ils ont vite trouve la solution. (They quickly found the solution.)

Common mistakes English speakers make

Building the adverb off the masculine when the adjective ends in a consonant: lentment or doucment are wrong, it's lentement and doucement from the feminine. Conversely, inserting a feminine -e when the masculine ends in a vowel: vraiement is wrong, it's vraiment. Spelling the -amment / -emment adverbs by ear - they sound the same, so you have to check the adjective ending (evident gives evidemment with -e-, constant gives constamment with -a-). And reaching for a -ment form where French has a suppletive: bonnement is not the everyday word for "well" - that's bien; "badly" is mal, not "mauvaisement".