An adjective is a word which acts to modify a noun in a sentence. While adjectives play a large role in many languages -- such as English -- many other languages have no adjectives at all. In English the set of adjectives is fairly well understood, though some people include other parts of speech -- such as articles like the -- in the class of adjectives.
There are two main roles an adjective may take in a sentence, and with a few exceptions each adjective is able to take either role just as easily. The first role is to act as a predicative adjective, in which the adjective modifies a preceding noun as a predicate, linked by a verb. An example of a predicative adjective can be found in the sentence: A zebra is striped. in which the adjective striped is linked the subject of the sentence, zebra, by use of the copula verb to be in the is form.
The second role an adjective may take is as an attributive adjective, in which it modifies a noun by being linked directly to the noun as part of the noun phrase. An example of an attributive adjective may be seen in the sentence: The striped zebra pranced. in which the adjective striped is directly connected to the subject of the sentence, zebra. In English, most attributive adjectives precede the noun they are going to modify, while in many Romance languages the adjective comes after the noun. So while in English we might say The beautiful woman. in French we would say Le femme jolie. which may be literally translated as The woman beautiful.
While most adjectives in English are able to be used just as easily either in an attributive or a predicative sense, there are some which are restricted to one role or the other. For example, the adjective sole can be used grammatically only as an attributive adjective, as can be seen in the sentence: This is the sole survivor. On the other hand, trying to use the adjective sole in the predicative role would result in the ungrammatical sentence: This survivor is sole. Other English adjectives, such as alone, may be used only as a predicative adjective, while attempts to use them attributively result in ungrammatical sentences.
Adjectives may be modified by adverbs or adverbial clauses, but not by other adjectives. Many adjectives, however, can easily translate into corresponding adverbs simply by adding the -ly ending to them. This can be seen in pairs such as quick/quickly and happy/happily.
In English and many other languages, adjectives also have a correct and incorrect order, depending on the type of adjectives used. Most native speakers learn this order instinctively, and related mistakes are one of the most obvious signs of a non-native speaker. For example, using the adjectives red, little, and two with the noun books, most native English speakers would intuitively order the adjectives to form the sentence The two little red books. To non-native speakers, however, it might seem just as intuitive to say The two red little books. or even The red two little books. both of which are immediately obvious as incorrect to a native English speaker.
As mentioned earlier, not all languages use adjectives; some use other parts of speech instead to fill this role. Many Native American languages, for example, use verbs to fill the role that adjectives play in English, so that rather than The woman is short. we are faced with something like The woman is shorting. Languages that use nouns as adjectives are often more comprehensible to speakers of English, since our sentence formations can easily allow for metaphoric description using only nouns, with a verb perhaps to flavor it, such as The sun was a blazing inferno. instead of The sun was hot. English also uses abstract nouns, for example to turn An important statement. into A statement of import.