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.
a unit
This u sounds like a consonant, it starts with “j”, so we use a.
an uncle
This u sounds like a vowel, so we use an.
Mind the pronunciation of the following word.
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.
My father is a car mechanic.
He is a good boy.
Bruce Springsteen is an American.
We need half a pound of sugar.
This is quite a good story.
He's such a good developer!