The indefinite article a / an is used only in the singular.
Hi is a nice boy.
Do you have a pen?
It’s a nice car, isn’t it?
Are you looking for a fork?
Can I get please, a cup of coffee.
The indefinite article is used before common nouns to indicate whole class.
A cow give us milk.
A dog is a faithful animal.
A bird has two wings.
Also you can use the indefinite article before hundred, thousand, milion, dozen instead of one.
A million and a half. = same as => One million and a half.
It’s about a thousand dollars.
A dozen eggs, please.
Before names of nationality to indicate one person, we also use the indefinite article.
In the following phrases we also use the indefinite article
One at a time.
In a short time.
For a change.
All of a sudden.
To be in a hurry.