There are two articles in English: “the” and “a / and” Articles shall be used for nouns.

Articles are divided into the following two types:
Indefinite Articles:          “a or an”
Definite Article:               “The”

The indefinite article “a / an” is used for common (non-specific) nouns.

Example: She bought a mobile.

In the above example, the ‘mobile’ is a common noun. The noun ‘mobile’ refers to a group of common things. It can be any mobile. It is not a specific or particular mobile.

On the other hand, the article “the” is only used for a specific or particular noun.

The article ‘the’ must be used before particular nouns such as ‘The America, The London, The Bible’. The article ‘the’ may also be used before a common noun, if the common noun is intentionally used as a particular noun in some context.

Example: He won the game.

In the above example, the article ‘the’ used before the noun ‘game’ shows that it was a specific or particular competition and not any game. The article ‘the’ is used to make the noun ‘game’ a particular noun. It means that this game was already discussed or known in the context,  in which, the above sentence is spoken.

Rules for the usage of Indefinite articles – “a  / an”

-The article “a” is used for countable (singular) nouns starting with a consonant or a vowel which sounds like consonant. e.g.  a car, a lamp, a university, a chair

-The article “an” is used for countable (singular) nouns starting with a vowel or consonant that sounds like vowel (mute in few words) e.g. an umbrella, an onion, an hour, an apple.

-Before a countable noun which refers to a major group of things.

e.g. A potato is rich in carbohydrates.

    1. For a name of some profession

e.g. He wants to become a doctor.
He is an engineer

    1. For expression of quantities

e.g. a few, a dozen, a couple, a lot

    1. For expression of numbers

e.g. a million, a thousand, a hundred

    1. Before a noun in exclamatory sentences

e.g. What a beautiful home!
What a nice book!

  1. Article cannot be used before uncountable nouns. e.g. milk, water.

Important note: If a word is countable (e.g. one book, two books), you must always use an article (or my, his, etc.):

I read a book.

I read book.

This is true even if there are adjectives before the noun:

He drives an old car.

He drives old car.

Never use or an with a word that is plural (e.g. trees, books) or uncountable (e.g. water, advice):

I asked her for advice.

I asked her for an advice.

Rules for the usage of Definite article – “The”

The definite article “The” is used for both singular and plural nouns in accordance with the rules given below. e.g. the car, the cars

-Before proper nouns which are particular and thought to be particular. e.g. the moon, the sun, the stars, the equator, the earth

-Before a noun which becomes particular by being discussed earlier, and now it is used again.

e.g. He gave money to a little boy and the boy bought toys for himself.

-For such a noun that has become specific in a phrase or a clause. e.g. the young energetic boy, the boy who helped us, the white shirt I bought,

-For superlatives or other adjective making the noun very particular one e.g. the most powerful, the first day, the only way, the best option,

-For a phrase which is a combination of a common and proper noun, e.g. The river Nile, The Muslim League Party, The New York City.

-For the names of the organizations. e.g. The Society for the protection of Child’s Rights,

See Also
Past Simple Tense
Countable and Uncountable Nouns