The French prefix a- is added to adjectives and nouns to create their antonyms; it's sometimes called the "alpha privative" (a privatif). The most common English equivalents are a- and non-.
The French prefix ci- can be added to various parts of speech to indicate the location or inclusion of something mentioned in a document (ci-joint, ci-inclus...)
The French prefix in- and its variants il-, im-, and ir- are added to adjectives, adverbs, and nouns to create antonyms. The English equivalents are un- and in-.
Adjectives comprise one of the eight French parts of speech, but certain members of other grammatical categories can sometimes be used as adjectives. These "non-adjectives" are invariable: there's no gender/number agreement with the nouns they modify.
The French prefix re- can be added to hundreds of verbs to make new verbs. Depending on the first letter of the verb it's added to, re- has a few variations as well as some different meanings.