Rule 1: The choise between a or an is determined bu sound. Before a word beginning with a vowel sound an is used.

An ass, an enemy, an inkstand, an orange, an umbrella, an hour, an honest man, an heir.

It will be noticed that the words hours, honest, heir, begin with a vowel sound, as the initial consonant h is not pronounced.

Rule 2: Before a word beginning with a consonant sound a is used.

A boy, a reindeer, a woman, a yard, a horse, a hole, also a university, a union, a European, a ewe, a unicorn, a useful article.

Because these words begin with a consonant sound.

Rule 3: Before word beginning with h and not accented on the first syllable, an is often used.

An historical novel, an hotel.

Rule 1: When we speak of a particular person or thing, or one already referred to.

The book you want is out of print.
I dislike the fellow.
Let us go to the club.

Rule 2: When a singular noun is meant to represent a whole class.

The cow is a useful animal.
The horse is a noble animal.
The cat loves comfort.
The rose is the sweetest of all flowers.
The banyan is a kind of fig tree.

Rule 3: With names of gulfs, rivers, seas, oceans, groups of islands, and mountain-ranges.

The Persian Gulf.
The Gangas is considered holy by the Hindus.
London is on the Thames.
The Red sea.
The Indian Ocean.
The Himalayas lie to the North of India.

Rule 4: Before the names of certain books.

The Vedas, The Puranas, The Iliad, The Ramayana.

Rule 5: Before common nouns which are names of things unique of their kind.

The sun, The sky, The ocean, The sea, The earth.

Rule 6: Before a proper noun only when it is qualified by an adjective or a defining adjectival clause.

The great Caesar : the immortal Shakespeare.
The Mr. Mishra whom you met last night is my uncle.

Rule 7:With Superlatives and ordinals.

The darkest cloud has a silver lining.
This is the best book of elementary chemistry.
He was the first man to arrive.
The ninth chapter of the book is very interesting.

Rule 8: Before musical instruments.

He can play the flute.

Rule 9: Before an adjective when the noun is understood.

The poor are always with us.

Rule 10: Before a noun to give the force of a Superlative.

The Verb is the word in a sentence.

Rule 11:As an Adverb with Comparatives.

The more the merrier.
The more they get, the more they want.