Use of a/an

Examples

1. Before a singular countable noun, when it is mentioned for the first time and represents no particular person or thing:

I can see a book on the table.

They live in a flat.

He bought an ice-cream.

2. Before a singular countable noun which is used as an example of a class of things:

A child needs love = All children need/Any child needs love.

3.With a noun complement. This includes names of profession:

My friend is a manager.

She’ll be a dancer.

4.With certain numbers.

Before half when half follows a whole number.

But ½ kg = half a kilo, though a + half + noun is sometimes possible.

With ⅓, ¼ , etc a is usual:

a dozen, a hundred, a million (but one dozen, one hundred, one million is also possible).

1 ½ kilos = one and a half kilos or a kilo and a half;

a half-holiday, a half-portion, a half-share;

a third, a quarter etc., but one is also possible.

In expressions of price, speed, ratio etc.:

5p a kilo, sixty kilometres an hour, four times a day; (Here a/an = per)

6. Before a singular countable noun after the word what in exclamatory sentences and after the words such, quite, rather:

Such a long queue! Such long queues! What a pretty girl! What pretty girls! She is still quite a child.

It is rather a difficult problem.

7. With the nouns in the function of apposition. But when the apposition refers to a well-known person the is used:

The report was made by Petrenko, a student of our University.

Jack London, the great American novelist, was born in San Francisco.

8. In a number of set expressions: a lot of, a great deal of, a good deal of, a great number of, a good many, a couple, a little, a few, at speed of, at a time hen, at a time, in time, on a large (small) scale, all of a sudden, by bus (train), to be in a hurry, to be in a position, to be at a loss, it’s a pity, for a short (long) time, as a result of, as a matter of fact, to have a good time, to have a mind, in a loud (low) voice, to have a look, to have a headache, to take a sit, to have a cold, to go for a walk.