We know that when speaking about singular indefinite countable nouns,
Examples:
I am a Person.
She is a good doctor.
That is an airplane.
I use a computer to write.
He lives in a big house.
Now let us look at the following rules of using “a”, “an"' and “the”.
Rule 1: When speaking aboutjobs, we normally use "a" and "an":
What is your father?
He is a businessman. (one of many businessmen)
What does your sister do?
She is a teacher. (one of many teachers)
What is your uncle?
He is the king. There is only one king!
Rule 2: When identifying something as part of a group, we use "a" and "an":
What is that?
What kind of horse is it?
It is a race horse.
What is a table?
A table is a piece of furniture.
Rule 3: When speaking about plural indefinite countable nouns, we use no articles:
We are people.
They are good doctors.
Those are airplanes.
I use computers to write.
They work in offices.
They live in big houses.
What are they?
They are businessmen.
What are those‘?
They are horses.
What kind of horses are they?
They are race horses.
Rule 4: We usually use "a" before words that begin with "h":
a history
a huge monster
a heavenly dessert
a house
But there are some exceptions. If the "h" is not pronounced, we use "an":
an hour
an heir
an honour
an honest man
Rule 5: We never use "a" or "an" with uncountable nouns.