Article : A, An

The definite article is used before singular and plural nouns when the noun is particular or specific.
The signals that the noun is definite, that it refers to a particular member of a group.


Compare the indefinite and definite articles in the following examples:
 
Indefinite (a or an)     
 
Singular 
 1.  a dog (any dog)                  
 2.  an apple (any apple)   
Plural
 1. some dogs (any dogs)        
 2. some apples (any apples)  

Definite (the)

Singular
1. the apple (that specific apple)
2. the dog (that specific dog)
Plural
1. the dogs (those specific dogs)
2. the apples (those specific apples)