They do not display any inflection; that is, their form does not change to reflect their precise role, nor any characteristics of what they modify.
In French, as in English, most adverbs are derived from adjectives.
In most cases, this is done by adding the suffix -ment ("-ly") to the adjective's feminine singular form.
For example, the feminine singular form of lent ("slow") is lente, so the corresponding adverb is lentement ("slowly"); similarly, heureux → heureusement ("happy" → "happily").
As in English, however, the adjective stem is sometimes modified to accommodate the suffix: Some adverbs are derived from adjectives in completely irregular fashions, not even using the suffix -ment: And, as in English, many common adverbs are not derived from adjectives at all: The placement of French adverbs is almost the same as the placement of English adverbs.