A, An and The are known as the articles. A and An are called the indefinite articles while The is known as the definite article.

When do we use these articles?

1) The indefinite article A or An cannot be used before things which we cannot count; as; ink, paper, sugar etc.

But we say:

a bottle of ink, a bag of sugar, a cup of tea, a piece of paper.

2) A or An can only be used with countable nouns in the singular; as, a book, an orange, a year, an hour

3) A is used before words beginning with a consonant, and An before words beginning with a vowel or silent h; as,

an umbrella, an old man, an honest man, a house, a pencil, a cow, a picture, a kilo of apples.

4) A, not An must be used before words which begin with a vowel sound of yoo. Such words are: Europe, European, uniform, union, unique, united, universal, university, usual.

Belgium is a European country. (not an European)
She hopes to be a university professor. (not an university)

5) The names of professions and occupations take the indefinite article.

My brother is an engineer. (not is engineer)
I hope to be a doctor. (not I hope to be doctor)

In the same way, such noun as hero, genius, tool, thief and liar, which describe someone by telling us the kind of person he is, take the indefinite article.

Beware of that fellow; he is a liar. (not……he is liar)

6) The indefinite article always follows the word such when it is applied to things that are countable.

I have known such a cold winter. (not a such cold winter)
Such an accident has never occurred before. (not such accident)

7) If an adjective is preceded by so, the a or an must be placed between the adjective and the noun.

I have never known such a cold winter. (not a so cold winter)

8) Adverbs such as only and just can be used before a few and a little, but not before few and little.

There were just a few mangoes left.
We have only a little money.

9) The definite article The is used when we speak of a particular person, thing or one already referred to.

I saw a boy and a girl riding on an ass. The boy was sitting behind the girl on the ass.
This is the boy who stole my purse.
Give me the blue cup.
He is the man for this job.

10) The is used when a singular countable noun is meant to represent whole class:

The cow is a useful animal.
The rose is the sweetest of all flowers.

11) The is used with superlatives.

Everest is the highest mountain in the world.
The darkest cloud has a silver lining.

Note, however, that most is sometimes used meaning very, as in ‘A most interesting book’ meaning ‘a very interesting book’.

12) The is used before adjectives and participles when these are used as plural nouns.

The rich ought to help the poor.
The cries of the dying could be heard.
The wounded were at once taken to hospital.

13) The is used before common nouns which are names of things unique of their kind:

The sun rises in the east.
The moon goes round the earth and the earth goes around the sun.

14) The is used in an adverbial way to show that two things increase in the same proportion:

The moon the merrier.
The sooner the better.
The more one has, the more one wants.

15) The, unlike A or An, is used before uncountable nouns to identify them:

The milk in this pot is meant for children.
The rice in this bag is not of good quality.

16) The is used before nouns which name the inhabitants of a country collevtively, not before the names of their languages:

The French live in France.
The inhabitants of France speak French. (not the French)
The Russians defeated the Germans. (but – Russians drink vodka.)

17) The is regularly used with:

Names of river; as:
Delhi stands on the Jamuna.
The Ganga is a sacred river.
Many ships use the Suez Canal. (not use Suez Canal)

Names of mountain ranges; as,
The Himalayas lies to the north of India.