The indefinite article

The indefinite article is a or an. The form an is used before a word that starts with a vowel sound.

a girl	a cat
an eight-year-old girl	an engineer

The indefinite article is used with singular countable nouns:

to refer to a person or a thing that you are mentioning for the first time in a conversation or a piece of writing.

A man was seen driving away in a black car.

to refer to a person or a thing which you do not want to be specific about.

I stopped off at a shop to buy a newspaper.
You go past a petrol station on the left, and then you’ll see our house on the right.

to refer to a person or a thing which you cannot be more specific about because there is not enough information.

A man called to see you this afternoon.
There was a telephone call for you a minute ago.

in definitions.

An octopus is a sea creature with eight tentacles.

when you refer to a person’s profession.

Her father is a dentist and her mother is a teacher.

to express a quantity, unless you wish to emphasize the number, when one must be used. The equivalent for plural nouns is some or no determiner at all. See Determiners.

I want a needle and a thimble.
Would you like a glass of wine?
There is only one glass of wine left in the bottle.
Guy has bought a skateboard.
We’ve got three pairs of rollerblades and one skateboard.

It is the sound, not the spelling, that decides where an is used. For example, although unique begins with a vowel, the sound at the beginning resembles a y- sound.

an idiot	an awful mistake
a unicorn	a unique experience

There are a few words that begin with a silent h-, in front of which an should be used. They are: heir, heiress, honest, honour, hour.

Very formal or old-fashioned speakers also use the an form with some words beginning with an h- that is not silent, especially historical and hotel.

I waited an hour.
They joined a historical society.
They joined an historical society.	(old-fashioned English)
They were staying at a hotel.
They were staying at an hotel.	(old-fashioned English)