INDEFINITE ARTICLE: A / AN

Use 'a' with nouns starting with a consonant (letters that are not vowels),
'an' with nouns starting with a vowel (a,e,i,o,u)

Examples:

A boy
An apple
A car
An orange
A house
An opera

NOTE:
An before an h mute - an hour, an honour.
A before u and eu when they sound like 'you': a european, a university, a unit

The indefinite article is used:

• to refer to something for the first time:
  An elephant and a mouse fell in love.
  Would you like a drink?
  I've finally got a good job.
• to refer to a particular member of a group or class

Examples:

• with names of jobs:
  John is a doctor.
  Mary is training to be an engineer.
  He wants to be a dancer.
• with nationalities and religions:
  John is an Englishman.
  Kate is a Catholic.
• with musical instruments:
  Sherlock Holmes was playing a violin when the visitor arrived.
  (BUT to describe the activity we say "He plays the violin.")
• with names of days:
  I was born on a Thursday

 

• to refer to a kind of, or example of something:
  the mouse had a tiny nose
  the elephant had a long trunk
  it was a very strange car
• with singular nouns, after the words 'what' and 'such':
  What a shame!
  She's such a beautiful girl.
• meaning 'one', referring to a single object or person:
  I'd like an orange and two lemons please.
  The burglar took a diamond necklace and a valuable painting.
  

Notice also that we usually say a hundred, a thousand, a million.

NOTE: that we use 'one' to add emphasis or to contrast with other numbers:
I don't know one person who likes eating elephant meat.
We've got six computers but only one printer.