English adjectives form a large open category of words in English which, semantically, tend to denote properties such as size, colour, mood, quality, age, etc. with such members as other, big, new, good, different, Cuban, sure, important, and right. Adjectives head adjective phrases, and the most typical members function as modifiers in noun phrases. Most adjectives either inflect for grade (e.g., big, bigger, biggest) or combine with more and most to form comparatives (e.g., more interesting) and superlatives (e.g., most interesting). They are characteristically modifiable by very (e.g., very small). A large number of the most typical members combine with the suffix -ly to form adverbs (e.g., final + ly: finally). Most adjectives function as complements in verb phrases (e.g., It looks good), and some license complements of their own (e.g., happy that you're here).

An adjective phrase (AdjP) is headed by an adjective and optionally takes dependents. AdjPs can take modifiers, which are usually pre-head adverb phrases (e.g., truly wonderful) or post-head preposition phrases (e.g., too big for you; afraid of the dark). The following tree diagram in the style of The Cambridge Grammar of the English Language shows the AdjP very happy to try, with the adverb very as a modifier and the clause to try as a complement.

English adjectives can take clauses, preposition phrases, and noun phrases as complements. Clause complements in adjective phrases can be either finite or nonfinite. Finite clause complements can be declarative (e.g., very pleased that I had bought his book) or interrogative (e.g., not sure whether I want to keep reading). Nonfinite clause complements can occur with a subject (e.g., happy for you to prove me wrong) or without a subject (e.g., eager to please). Adjectives that take preposition phrase complements license preposition phrases headed by fixed prepositions. For example, dependent takes preposition phrase complements headed only by on or upon. In some cases, a complement is obligatory; I'm loath to admit it is fine, but *I'm loath is incomplete. A small number of adjectives (due, like, near, unlike, and worth) can take noun phrases as complements. For example, worth can function as the head of an adjective phrase with a noun phrase complement (e.g., worth a second chance).

The prototypical pre-head modifiers of English adjectives are adverb phrases headed by degree adverbs, such as very and too. For example, the adjective tall can be modified by the adverb phrase very. Less common pre-head modifiers in adjective phrases are noun phrases (e.g., six feet long), preposition phrases (e.g., by no means realistic), and determiner phrases (e.g., that small).

Preposition phrases function as post-head modifiers in English adjective phrases. In the adjective phrase foolish in the extreme, for example, the preposition phrase in the extreme functions as a modifier. Less commonly, certain adverbs (indeed and still) and one determiner (enough) can head phrases that function as post-head modifiers in adjective phrases (e.g., very harmful indeed, sweeter still, and fair enough).

While adjectives themselves function only as heads in adjective phrases (an AdjP is often a head adjective with no dependents), adjective phrases function at the clause level as predicative complements and predicative adjuncts. At the phrase level, adjective phrases function as modifiers and predeterminatives in noun phrases and complements in some preposition phrases (e.g., they didn't look as good).

At the clause level, adjective phrases commonly appear as predicative complements. A predicative complement is a constituent that ascribes a property to a predicand. For example, The dinner was lovely ascribes the property of being lovely to the dinner, the syntactic subject and semantic predicand. Predicative complements may be subject-related, as in the previous example, or object-related, the latter being licensed by complex transitive verbs such as feel and make, as in That made her hungry, where the property of being hungry is ascribed to the syntactic object and semantic predicand, her.

Adjective phrases also function as predicative adjuncts in clause structure. Such adjective phrases can be integrated into the clause (e.g., Love dies young) or detached from the clause as a supplement (e.g., Happy to see her, I wept). Adjective phrases functioning as predicative adjuncts are typically interpreted with the subject of the main clause being the predicand of the adjunct (i.e., "I was happy to see her"). When this is not the case, such supplements are often deprecated as dangling modifiers.