A and An are called Indefinite Articlees because they are used when we do not specify which particular person or thing we are referring to; the person or thing remains indefinite, i.e not specifically defined.
Use of the Indefinite Article
1. A and An are weakened forms of one. So their primary use is in the sense of one:
I bought a dozen oranges.
He has an ulcer on his leg.
We shall be leaving in a day or two.
A bird in the hand is worth two in the bush.
A stitch in time saves nine.
2. It is used sometimes in the rather vague sense of a certain:
I saw a hawker selling his wares in the street.
A man came and knocked at the door.
In this letter a correspondent pleads for the use of the Roman script for all Indian languages.
3. It is occasionally used in the sense of any or a single:
At this rate there won't be a lion left in the Gir forest in a few year's time.
There is not a man here who will not support you.
He did not speak a word in self-defence.
4. It is also used in the sense of every:
He gets a salary of Rs.2000/- a month.
He makes thirty thousand a year.
Tennis balls cost Rs.108/- a dozen now.
5. It is sometimes used with a Singular Noun to make the latter represent a whole class:
A dog is faithful to his master. (meaning all dogs)
Only an infantry man knows what war is. (all infantry men)
A parrot can repeat what you say. (all parrots)
(Note. This use is similar to that of the Definite Article for the same purpose. ‘Compare', ‘The dog is faithful to his master'.)
6. The Indefinite Article is occasionally used before a Proper Noun to make it a Common Noun:
Here is a Daniel (a wise and upright judge) come to judgement.
He is a Valmiki (aninspired poet).
Why should you worry? You are a Rockfeller (a millionaire).
He is a second Newton (a great scientist and philosopher).