12.1.1 The indefinite article (A, AN)
The indefinite article is generally used before a singular countable noun when it has not been mentioned before and does not represent a unique thing or person:
A man and a woman came in.
When the noun is mentioned again, the definite article the is used:
A man and a woman came in. The man was about 40 years old.
A is used before a word beginning with a consonant sound, but if the following word starts with a vowel sound, we use an:
Regina was wearing a dark skirt and an orange blouse.
My uncle is a university lecturer. (university starts with the vowel u but the consonant sound /j/)
He is an honest man. (honest starts with the consonant h but it is silent here)
The indefinite article with a member of a class
The indefinite article is used before a noun when it refers to any member of a class of things:
A real friend is always by your side. (Real friends are always by your side.)
The indefinite article in expressions of quantity
The indefinite article is used in certain expressions of quantity:
I'm having a lot of problems with Michael.
I think a great many people will agree with me on that.
The indefinite article with certain numbers and fractions
The indefinite article is used with certain numbers and fractions:
a/one hundred, a/one thousand, a/one million
one and a half kilos / a kilo and a half
a/one third
a/one quarter
a dozen
a quarter of an hour
half a minute
The indefinite article instead of PER
In expressions of measurement, the indefinite article can replace the more formal per:
He was driving at 80 miles an hour when the car skidded.
"How much are those tomatoes?" "Three euros a kilo."
I go to the sauna twice a month.
The indefinite article to describe and classify something
The indefinite article is used before a noun which describes or classifies something:
That was a lovely dinner!
Mary is a teacher.
The koala is a marsupial.
The indefinite article with certain illnesses
The indefinite article is used with names of certain illnesses:
Is it a cold or just an allergy?
I have a splitting headache.
But:
The flu is a virus that can make you sick for a week or longer.
(The) measles is best known for its characteristic skin rash.