There are three articles: a, an and the.

(i) ‘a’ and ‘an’ are referred to as indefinite articles. This article is used before:

1. A singular noun which can be counted.
2. Singular countable noun which is used to represent a class or group of things or people.
3. When giving somebody’s occupation 

Examples:

1. Jane has bought a new pen.
2. A baby needs a lot of sleep

1. He is an electrician.

A and AN

(ii) ‘The’ is referred to as the definite article. It is used before:

1. Singular and countable nouns.

2. Nouns representing things there is only one of.
3. An adjective representing a class of people or things.
4. Superlatives.

Examples:

1. The people of Kenya are friendly.

1. The cave was frighteningly dark.

1. The dead should be respected.
2. Mt. Kenya is the highest mountain in the country.

THE IS USED:

1. to refer to something which has already been mentioned.

2. when both the speaker and listener know what is being talked about, even if it has not been mentioned before.

3. in sentences or clauses where we define or identify a particular person or object:

4. to refer to objects we regard as unique:

5. before superlatives and ordinal numbers:

6. with adjectives, to refer to a whole group of people: the Japanese , the old.

7. with names of geographical areas and oceans: the Caribbean, the Sahara, the Atlantic.

8. with decades, or groups of years: she grew up in the seventies.

Here is a video on the articles A, An and The

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iii) No article is needed when:

1. Plural nouns are referring to people or living things in general.
2. Before an abstract noun when used in a general statement.

Examples:

1. Furniture is expensive.

1. Honesty is the best policy.

The Article: