One of the eight parts of speech, adjectives are a type of modifier. They serve the same purpose in French and English, but they are very different in two respects.
By definition, adjectives modify nouns. But more than 30 French adjectives can sometimes modify verbs instead, thus taking on the role and characteristics of adverbs, including the fact that they are invariable.
There are six French adjective / pronoun pairs, where an adjective + noun can be replaced by a corresponding pronoun.
When describing someone as capable of doing or determined to do something, a preposition is required between the adjective and verb. In French, the choice of preposition depends on the adjective that precedes it, not the verb that follows.
Adverbs of manner express how the action of a verb occurs. In English, the vast majority of adverbs of manner end in -ly, whereas in French, they mostly end in -ment. They are created from adjectives.
Grammatical agreement is a vast topic - and one of the banes of French students. While in English we have a few pronouns and adjectives that indicate gender and number (e.g., he/him/his and she/her/hers), in French, agreement is found in 5 of the 8 parts of speech.
The three French verb constructions which include some form of être plus a past participle usually require grammatical agreement of the past participle with the subject.
Ce, cette, cet, and ces are demonstrative adjectives, which are used to indicate a specific noun or nouns. In French, they must agree with the noun(s) in number and sometimes gender.
Determiners are a category of grammatical terms that includes articles, numbers, and non-qualifying adjectives. Unlike qualifying adjectives, determiners serve two functions: they introduce and modify nouns at the same time.
French grammar is sometimes trumped by pronunciation, as in the case of euphonic adjectives. Because French does not like the hiatus created when a word ending with a vowel precedes a word that begins with a vowel or mute h, a few adjectives change their spelling—and thus their pronunciation—for purely euphonic reasons.
To express admiration, surprise, contempt, or another strong feeling about a noun, you can use the exclamative adjective quel, meaning "what (a)."