French adverb formation with the prefix -ment
A large number of adverbs can be formed by adding -ment to an adjective, in a similar way to the use of the suffix -ly in English.
Generally, -ment is added to the feminine form of the adjective1, particularly where the feminine form differs in pronunciation from the masculine (so generally where the masculine form ends in a consonant in the spelling). For example:
The adjective frais ("cool", "fresh") has the feminine form fraiche, which traditionally is spelt with a circumflex: fraîche. In both the feminine form and the adverb, spellings with and without the circumflex are acceptable: fraichement (new spelling proposed in the 1990 spelling reform) or fraîchement (traditional spelling).
Adjectives ending in -ant and -ent
These have the slight irregularity that in general:
Note that in either case, the suffix is pronounced as though written -amment (i.e. in -emment, the first e is pronounced like an a).
Common examples include: évident ("obvious") > évidemment ("obviously"); récent ("recent") > récemment ("recently"), fréquent ("frequent") > fréquemment ("frequently"); apparent ("apparent") > apparemment ("apparently"); constant ("constant") > constamment ("constantly"); courant ("common", "current") > couramment ("commonly", "fluently").
Exceptions: lent > lentement, présent > présentement, plus (rarer) adjectives ending in -ément: clément > clémentement, véhément > véhémentement.
Addition of -ment to the (written) masculine form
When the adjective ends in a single vowel in the spelling, no -e is added when forming the adverb. Note that this is really just a spelling issue: there is no pronunciation difference between the masculine and feminine forms of these adjectives. Common examples include absolu > absolument ("absolutely") and poli > poliment ("politely"), plus some adverbs formed from regular past participles: désespéré > désespérément ("desperately").
The adjective vrai also follows this rule, giving vraiment.
In a few cases of adjectives ending in -i or -u, traditional spelling adds a circumflex accent (e.g. continu > continûment). However, since the 1990 spelling reform, it is acceptable to omit the circumflex, so that various adverbs treated as "irregular" in pre-reform grammar books can now be treated as regular if you are happy to adopt the new spelling (assidument, continument etc).
Adverbs changing -e to -é
A handful of adjectives change -e to -é when forming the corresponding adverb. Note that many of these are quite formal or literary words:
plus the following:
There are a handful of adverbs that we might regard as "irregular" (note that some of these are just spelling issues, though):
1. The suffix -ment used to form adverbs derives from the Latin word mens, mentis meaning "mind" (and which survives in words such as mental and mente, still the Spanish word for "mind"). Thus originally, the notion of calmly was expressed with a phrase meaning "with a calm mind". The word mens was feminine, hence the feminine form of the adjective was used. Today, -ment is perceived as a suffix rather than a word in its own right. On the other hand, an interesting hangover of this suffix in Spanish (-mente) is that it is given an extra syllable stress (so that Spanish adverbs ending in -mente have two stressed syllables), whereas all other Spanish words have a maximum of one stressed syllable in normal non-emphatic speech.