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The Indefinte Article (“a/an”)

We use the indefinite article “a/an” with singular countable nouns: 
a boy, a girl, a cat

The indefinite article has no plural form. 
a boy — boys

We use an if the following word starts with a vowel sound.
 
the following word starts with a consonant  the following word starts with a vowel
a boy an aunt
a school an old school
a girl an American girl
 a university an honest man

 

Mind the pronunciation of the following word.
a unit an uncle
 This u sounds like a consonant, it starts with “j”, so we use a.  This u sounds like a vowel, so we use an.
 
 

Use of the Indefinite Article “a/an”

- before phrases of time and measurements (per week/weekly)
We have English 4 times a week.
I go on holiday twice a year.
Our car can do 220 kilometres an hour.
Tomatoes are $2 a kilo.

- before jobs
My father is a car mechanic

- with a noun complement
He is a good boy.

- before nationalities
Bruce Springsteen is an American.

- half/quite/such + noun
We need half a pound of sugar.
This is quite a good story.
He's such a good developer