Практическая грамматика

a beautiful red rose, some English books, his new black suit.
The indefinite article has the forms: a and an. The form a is used before words beginning with a consonant sound (a book, a pen, a student). The form an is used before words beginning with a vowel sound (an opera, an apple, an hour).
The definite article has one graphic form the.
The indefinite article has developed from the Old English numeral an (one), and as a result of its origin it is used only with nouns in the singular.
The definite article has developed from the Old English demon­strative pronoun se and in some cases it has preserved this demon­strative meaning in Modern English.
The use of the indefinite article implies that the object is pre­sented as belonging to a class.
The use of the definite article shows that a particular object is meant.
The absence of articles with class nouns in the plural, with abstract nouns and nouns of material have grammatical significance: it shows, that the nouns are used in a general sense.
With nouns in the plural some is often used. Some, as well as the absence of articles with class nouns in the plural, is the equiva­lent of the indefinite article in the singular. Some is used when the speaker wants to emphasize the idea of number. Some is also used with nouns of material if the idea of quantity is implied. Some has the meaning of “several” with class nouns and “a little” with nouns of material. Some is hardly ever translated into Russian.


§ 2. Functions of the Article.

The articles have morphological, syntactic and communicating functions.

The morphological function of the articles consists in serving as formal indicator of the noun: the presence of the article signals that what follows is a noun.
The articles have two syntactic functions:

1. 
   The article separates the noun phrase from other parts of the sentence:
   

a magazine.
John has bought an interesting magazine.
an interesting English magazine.

2. 
   The article may connect sentences within a text by correlating a noun it modifies with some word or group of words in the previous context:
   

John has bought a book. The book is interesting.
Thus, the article in such a case has the connecting function.
The articles also have the communicating function.
A noun with the indefinite article may introduce new information in the sentence: it is then the focus of communication:
A pretty girl of about eight ran into the room.
A noun with the definite article in the initial position usually indicates given information and is not the focus of communication: