Adjective phrases consist of adjectives together with elements which complement or modify them in different ways. Although adjective phrases are potentially complex, in practice most of them have a fairly simple structure. Thus, a typical adjective phrase consists of a head in the form of an adjective sometimes accompanied by degree modifiers, as in the following example:
(1) The poor living conditions make planning your future [almost impossible].
The adjective phrase almost impossible consists of the head impossible and the degree modifier almost.
The function of adjective phrases
Adjective phrases have two primary functions. First, they can be used to modify nouns inside noun phrases, as in the following example:
(2) The scarcity of supplies has become [an increasingly difficult problem].
Here, the noun phrase within square brackets has the noun problem as its head, and the adjective phrase increasingly difficult serves as a modifier. This function of adjective phrases is referred to as attributive.
The second main function of adjective phrases is as predicatives in clause structure, following verbs like be, become, seem, etc.
(3) Maintaining a reasonable level has become increasingly difficult.
This function of adjective phrases is referred to as predicative.
Whether it is attributive or predicative, an adjective phrase always modifies (i.e. somehow provides more information about) a noun phrase (or a dependent clause functioning as the subject of a sentence, e.g. To be a true adult is sometimes difficult.).
Some adjectives only occur in predicative adjective phrases. Most dictionaries mark such adjectives as special (e.g. by labelling them "predicative only" or "not in attributive use"). A few examples are given here:
Bill is afraid of dogs.
NOT: *Bill is an afraid boy.
The two brothers are very much alike.
NOT: *The two most alike results were compared.
We are aware of the difficulties
NOT:*We expect more aware attempts in the future.
The ungrammatical examples in the right-hand column illustrate the fact that these adjectives cannot be used attributively (i.e. to modify nouns in noun phrases).
Adjectives as heads of noun phrases
A somewhat odd function of adjectives is that of serving as heads of noun phrases. In English, this use is virtually restricted to noun phrases that have generic reference (i.e. which refer to entities in general, rather than to specific instances). The following two examples illustrate this use:
(4) We must plan for the future needs of the elderly. [elderly people in general]
(5) In the early 20th century, the study of the supernatural was attracting a lot of interest. [supernatural phenomena in general]
In the first example, reference is made to a category of people who share some characteristic (age in this case). In the second example, reference is made to an abstract concept.
Almost without exception, these two uses are the only ones where adjectives are used in this way in English. When reference is made to a specific individual or a specific group of individuals or specific instances of abstract concepts a nominal head (a noun or a pronoun) is used.
(6) The elderly woman was confused and disoriented.
(7) They insist that the supernatural events described in the Bible are real-world manifestations of God.