There are three articles in English: A, An and The. They are called articles as they have significant parts of speech. They are also called Determiners because they affect the meaning of noun.
There are two types of articles:
1. Indefinite Articles 
2. Definite Article

1. Indefinite articles 
A or An is called the Indefinite Article because it is used when we do not speak of any particular or definite person or thing.
eg: I saw
a boy in the bazaar.
I ate
an apple.

(A)  Correct use of 'A'
'A' is used before:
(1) Words beginning with the sound of a consonant. eg: a girl, a book, a crow
(2) Such vowels as have the sound of 'yu'. eg: a useful person, a university, a European
(3) 'O' when it is sounded as 'wa'. eg: a one-rupee note, a one-way traffic, a one-eyed giant

(B) Correct use of 'An'
'An' is used before:
(1) Words beginning with a vowel sound, a silent 'h' when the accent is not on the first syllable. eg: an eye, an honourable man, an heir, an army

2. Indefinite Article
The is called the Definite article because it is used when we speak of some particular person or thing.
eg: This is the boy whom I saw in the bazaar yesterday.

Correct use of 'the':
(1) before any noun when we want to particularise it:
eg: Close the window on your left.
(2) with superlatives:
eg: Chandrapal is the most intelligent child in the class.
(3) with words like first, last, next, only:
eg: Nikhilesh is the only son of his parents.
(4) with such nouns as earth, moon, sun, sky, etc:
eg: The earth revolves around the sun.
(5) with a common noun when that noun represents the whole class:
eg: The dove is a very gentle bird.
(6) with the names of well-known buildings:
eg: We visited the Akshardham temple last month.
(7) with the names of rivers, seas and oceans:
eg: The Godavari joins the Bay of Bengal.
(8) with the names of newspapers:
eg: The Hindu is a widely circulated newspaper in India.
(9) with the names of mountain ranges:
eg: The Alps is one of the longest mountain ranges in Europe.
(10) with the names of countries if the name includes a common noun:
eg: The U.K and the U.S.A are developed countries.
(11) with the names of holy books:
eg: The Mahabharata was written by Vyasa.
(12) with the names of trains, ships, aeroplanes, etc:
eg: We travelled by the Avantika Express.
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