a/an is used:
Before a singular noun which is countable (i.e. of which there is more than one) when it is mentioned for the first time and represents no particular person or thing:
I need a visa. They live in a flat. He bought an ice-cream.
Before a singular countable noun which is used as an example of a class of things
A car must be insures =
All cars/Any car must be insured.
A Child needs love =
All children need/Any child needs love.
With a noun complement. This includes names of professions:
It was an earthquake. She'll be a doctor. He is an actor.
In certain expressions of quantity:
a lot of a couple
a great many a dozen (but one dozen is also possible)
a great deal of
With certain numbers:
a hundred a thousand
Before half when half follows a whole number:
1 1/2 kilos = one and a half or a kilo and a half
but 1/2 kg = half a kilo ( no a before half), though a + half = noun is sometimes possible:
a half-holiday a half-portion a half-share
In expression of price, speed, ratio etc.:
5p a kilo sixty kilometres an hour
10p a dozen four times a day
(Here a/an = per.)
In exclamations before singular. countable noun :
Such a long queue! What a pretty girl! but
Such long queues! What pretty girls!
a can be placed before Mr/Mrs/Miss + surname:
a Mr Smith a Mrs Smith a Miss Smith
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