Predicative Adjective

Definition: A predicative adjective is not part of the noun phrase headed by the noun it modifies; rather, it is the complement of a copulative function that links it to the noun. A predicative adjective comes after a copula verb (linking verb) and not before a noun.

For example:

The book is big.
* The predicative adjective "big" is linked by the verb "is" (which contains the copulative function) to the noun book, which it modifies

There are a few adjectives which cannot occur in both predicative and attributive position. Some only occur in attributive position (they can't function as a predicate). Examples include "main" and "former".

For example:

This is the main reason.
* This reason is main. (ungrammatical)
This is the former president.
* This president is former. (ungrammatical)

A few other adjectives can only be predicative, i.e. they can't occur in attributive position. An example of this is "alone":

This man is alone.
* This is an alone man. (ungrammatical)