Definitions 

The adjective is a word used to limit or qualify a noun.

For example

good schools
diligent boys
this table
ten men
the box

    
All words with the construction of the adjective are considered under the heading of adjectives. The article, like the adjective:

• belongs to the noun.
• has the same construction as the adjective.

It is for these reasons that the article is placed among adjectives.
Every adjective is a depend, or subordinate word, and must belong to some noun or pronoun as its principal.  When the noun or pronoun to which the adjective belongs has been previously used in the same sentence, or when the noun or pronoun is some indefinite word such as person, someone, or something, the noun or pronoun, itself may not appear.

For example

I will give you this book, if you give me that [book].
The kingdom of heaven sufereth violence, and the violent [person] take it by force.

An adjective belonging to an understood or omitted noun takes the place of that noun and can be said to be an adjective used as a noun.

Classes of Adjectives

Adjectives are divided into two classes:

• limiting adjectives 
• qualifying adjectives