An adjective is a type of word in a language that is used to describe things or people. Words like “hot,” “sweet,” “boring,” and “beautiful” are adjectives.
Usually, in English, adjectives come before the thing they describe—a hot day, a sweet pickle—but sometimes they come later in the sentence, as in “I thought today’s class was really interesting.” (The adjective “interesting” describes the class.) In certain cases, an adjective can even come right after the word it describes. For example, we don’t say “I want a good something to eat.” Instead, we say “I want something good to eat.”
Adjectives are usually one-word units, but sometimes they have two or more words separated by a hyphen as in four-wheel-drive vehicle and cancer-related research.
Most adjectives have what are called comparative and superlative forms; that is, there is a basic form like “cold” and a form like “colder” that we use when we compare how cold things are. This form with “-er” is called the “comparative” form. Then there is another form like “coldest” that describes something that is colder than any of the other things we are talking about. The form with “-est” is called the “superlative” form. Some adjectives do not have these special forms with “–er” and “-est,” however, and we use the words “more” and “most” with them instead; for example, we say “more interesting,” “more level-headed,” “most delicate,” “most hilarious.”* If a word has a form with “–er” or “–est,” or if it makes sense to use “more” or “most” with it, then we can be sure that it is an adjective. Still, some words that are adjectives do not have comparative or superlative forms at all, so the main criterion for classifying them as adjectives is that they modify nouns; that is, they describe words that represent things or people.
*There isn’t a perfect rule about which adjectives must use “more” or “most” rather than “–er” or “–est” forms, but there are some general tendencies. The shorter the word is, the more likely there will be a special form with “–er” and “–est.” Most one-syllable adjectives act this way and many two-syllable adjectives do too, especially if they end in “–y” (e.g., “sleepy,” “sleepier,” “sleepiest”). If the word has three or more syllables, we always use “more” and “most.” If the adjective ends in a suffix, such as “–ful,” ”-ing,” “-ed,” or “-less,” we use “more” and “most,” even if it’s a two-syllable word. Adjectives made up of hyphenated words always use “more” and “most.”