INDEFINITE ARTICLES A / AN

1 - We use A before a word beginning with a consonant sound.

A cat
A chair
A new car
A house
A university
A new car

2 - We use AN before a word beginning with a vowel sound.

An animal
An elephant
An old computer
An hour

3 - We use A / AN:

* Before a singular countable noun when it is the first time we mention it

He lives in a house

* Before certain numbers

A hundred
A thousand
A million

* With professions

He is an actor.
She is a dentist.

4 – We can’t use A / AN

* Before plural nouns
Dogs are lovely animals.

* Before uncountable nouns like milk / advice / furniture / sugar
Would you like some milk?

* Before names of meals (breakfast / lunch / tea / dinner)
We have lunch at 12 o’clock.

Note: we use a + adjective + meal
We had a good breakfast.