An adjective is something like "a word that describes a noun." If asked to name some adjectives, you might come up with words like small, pretty, and blue - in other words, descriptive adjectives. But did you know that there are many other types of adjectives as well?
One of the eight parts of speech, adjectives are a type of modifier; that is, they modify or describe nouns in a certain way, letting you know the size, shape, weight, color, nationality, or any of a myriad other possible qualities of nouns.
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.
In both French and English, many adjectives can be used as nouns as a sort of shorthand to reference what you'd otherwise need an adjective + noun to refer to.
At first glance, adjectives and pronouns might seem to have very little in common, since adjectives are used with nouns, while pronouns replace nouns. But there is an interesting relationship that can help you use both parts of speech more effectively: 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 usually 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.