The typical adjective is able to function as a complement after the verb be. The following examples have the adjectives underlined.
The festival is lively
His voice was harsh
This problem has been difficult
No man is infallible
Your behaviour was foolhardy
The music is loud
The materials in use are brittle
Those potatoes were hot
When an adjective occurs in this position it is said to be predicated of the subject: lively is predicated of the festival, brittle is predicated of the materials in use, and so on. This function of adjectives is therefore said to be the predicative function.
It is not only adjectives that can be predicated of a subject. Nouns also can be used in this way, as in The festival is an expense, This problem has been the stumbling block and The men are students. So how can we tell the difference between an adjective and a noun? It is partly a question of what happens when an adjective phrase contains more than a head. The words that can act as modifier to an adjective head are different from those that can act as modifier to a noun head:
His voice was very harsh
This problem has been somewhat difficult
The potatoes were terribly hot
Some tigers are quite docile
The words acting as modifiers to the adjective heads in these examples are very, somewhat, terribly and quite. The name for this kind of modifier in adjective phrases is intensifier. In noun phrases the noun head may have a determiner, while adjective heads cannot have determiners. We can say every chair but not *every hot; and we can say terribly hot but not *terribly chair. Figure 16 shows the structure of the adjective phrase.
Furthermore, nouns can be inflected for plural number—at least, countable nouns can—while adjectives cannot. *The men were hungries is not a possible sentence, while The men were students is all right.
It is not only the verb be that can take an adjective phrase as its complement. There are some other verbs that can do so: