Pneumonoultramicroscopicsilicovolcanoconiosis

When to Use “A”
‘A’ is used before words beginning with a consonant sound. For example:

A man
A hat
A lamp
A teacher
A cat
A book

When to Use “AN”
The article ‘an’ should be placed before words that begin with a vowel sound. The initial sound should be a, e, i, o, or u. For example:

An apple
An egg
An island
An article
An umbrella
An hour

A vs. An | 13 Rules for Using Indefinite Articles
Rule #1:

‘A’ must be used before words which begin with a vowel symbol pronounced with the same sound as the ‘y’ or a ‘w’-like sound. For example:

A Euro
A unicycle
A union,
A unique
A united
A university
A user
A unicorn

Rule #2:

Sometimes, ‘an’ is found before words beginning with the letter ‘h’.

Silent ‘h’ : ‘an’ is used before these words. For example:

An hour
An honor
An heir
An heirloom
An hourly
An honorarium
An honesty
An honorary

But this practice is going out. If ‘h’ is pronounced, ‘a’ is used:

A hamburger
A hall
A hat
A hacksaw
A horse
A house
…
Rule #3:

We use ‘an’ before abbreviations that begin with vowel sound. For example:

An M.A.
An M.Sc.
An M.B.B.S.
An M.L.A.
An N.C.C Officer
An M.Com.
An M.C.A
An M.B.A
…
Rule #4:

A or an must be used before a singular noun standing for things that can be counted. For example
He was sitting in a chair.
She had a bottle in each hand.
Did you bring an umbrella?
I would like an apple.
Rule #4:

A or an must be used before the names of professions. For example:

She is a director.
Peter is an actor.
Rule #6:

Words like hero, genius, fool, thief, and liar take the indefinite article. For example:

We all regarded him as a hero.
I suspect he is a thief.
Rule #7:

The following word has indefinite article. Such + a/an + noun (or) Such + a/an + adjective + noun. For example:

Such a policy
Such a long trip
Such a great teacher
Such an attractive woman
Rule #8:

The following word has indefinite article. so + adjective + a/an + noun. For example:

So boring a book
So beautiful an actress
Rule #9:

‘Few’ and ‘Little’ are negative meaning. ‘A few’ and ‘a little’ are positive and mean ‘some‘. For example:

He is young and has little experience of life.
Sweeten the mixture with a little honey.
He had few coppers in his pocket.
I went to spend a few days at the seaside.
Rule #10:

In its original numerical sense of one. For example:

Not a word was said.
Twelve inches make a foot.
A bird in the hand is worth two in the bush.
I have a novel with an intricate plot.
Rule #11:

A an = one thing or person. For example:

She works in a hospital.
There’s a problem with the drains.
I’ve got a puppy.
There was a girl sitting next to him.
Rule #12:

We use a an when we say what a thing or a person is. For example:

“Coca Cola” is a beverage.
Badminton is an indoor game.
A dog is a domestic animal.
Mary is a shy girl.
Rule #13:

We use a an for jobs, etc. For example:
She works as a nurse in a hospital.
John is a fireman.
I want to be a doctor.
She is a cashier in a bank.
Note: The indefinite articles are not used before plural nouns, proper nouns, abstract nouns, uncountable and material nouns. For example:

A dog is a faithful animal.
Dogs are faithful animals.