Both articles a and an are used:
before a singular noun when it is mentioned for the first time and represents no particular person or thing:
I need a passport.
They live in a house.
I have to buy an umbrella.
also before a noun which is used as an example of a class of things.
A child needs love.
A responsible person must be insured.
a and an is omitted before
plural forms (Cats are extremely independent.)
uncountable nouns (Do you need money?)
names of meals (I have breakfast at 7 a. m.)
the is used for singular and plural
1) the is used before a noun which has become definite as
a result of being mentioned a second time:
John has bought a car. The car cost him 25, 000$
2) the is used before a noun which has become definite by the addition of a phrase or clause:
the woman in black
the girl I am in love with
3) the is used before a noun which can represent only one particular thing:
Mum is in the kitchen.
Pass the salt, please.
4) the is used before superlatives and first, second etc.
the first year of…
the best friend
5) the is used before a noun to represent a class of people, animals or things:
The dolphin is in danger of becoming extinct.
The television has completely changed our lives.
The small shopkeeper is finding life more and more difficult.
(however, man, used to represent the human race, has no article)
6) the + adjective represents a class of people
7) the is used before certain proper names of seas, rivers, groups of islands, chains of mountains, plural names of countries, deserts, regions:
8) the is used before names consisting of noun + of + noun
the Cape of Good Hope
the United States of America
9) the is used before musical instruments:
The definite article the is not used :
before abstract nouns (Men fear death.)
before names of games ( I can’t play tennis.)
in a number of common expressions:
to school at school from school in/to class
to/at/from college/university to/in/from church
to/into/in/out of prison/hospital/bed to/at/from work
to/at sea to/in/from town at/from home
for breakfast at lunch to dinner at night
on foot to go to sleep
when the above expressions are used with articles,
they have special meanings:
He is in prison. (as a prisoner)
He is in the prison.(perhaps as a visitor)
To sum it up remember three golden rules:
1) Do not use the to talk about things in general (NOT the life is hard).
2) Do not use singular countable nouns without articles (NOT I have car).
3) Use a/an to say what people’s jobs are (NOT He is teacher).