article

1. The Definite Article
How simple English is !!  There is only one definite article, and that is "the"; the only difficulty is knowing when to use it, and when it is not needed.
The basic rules:
Basically, the rules are quite simple:
*  The definite article shows that a noun is being used in a "defined" or restricted context.
*  When a noun is used in a non-defined or "generalizing" context, a definite article is not needed.
Two sorts of noun.
Use of the definite article therefore depends on the nature or type of noun that is being used.
As in other languages, nouns in English can be divided into two distinct categories, called:
 count nouns and non-count nouns.

► Count nouns are nouns referring to items that can be counted, for example:
 One car, two pencils, three people, four guitarists, five hotels etc.
Count nouns can be used in the plural.

►Non-count nouns are nouns referring to abstractions, substances or generalizations, or example:
 Oxygen, health, money, heat, astronomy
Non-count nouns can NOT usually be used in the plural.

THE BIG QUESTION:
Sometimes it is not easy to decide if a plural noun is being used as a generalization, or in a restrictive context: often the speaker or writer can choose. Look at these two sentences:
 a) London buses are red
 b) The London buses are red.
In example (a), the writer is clearly implying a generalisation on the noun group "London buses": All London buses are red.
In example (b), he is referring to a restricted or defined category of the noun "buses",  London buses are red, but other buses may be of other colours.
In cases like these, one therefore has a choice; but the choice is not always completely free, as it often depends on context. Is it more important or more logical to imply a generalization, or a limitation? 
Deciding whether it is best to use an article in such cases is a skill that has to be mastered!
► Follow this link for more information on count and non-count nouns
2. The Indefinite Article
English has two indefinite articles, a and an
     a is used before nouns starting with a consonant or a semivowel
     an is used before nouns starting with a phonetic vowel

Examples:  a dog,  a cat,  an apple,  an orange, but a university (because the word university does not start with a phonetic vowel)

Indefinite articles are used when a count noun in the singular refers to a non-specified entity
Examples:  A train ,  An elephant  (meaning any train, or any elephant).

There is no inderfinite article in the plural. The word "some" is occasionally said to be a plural indefinite article, but really it is a quantifier (like many, few, etc.)

By definition, plural nouns refering to non-specified entities are generalisations, therefore need no article.