Articles

The definite article (the) is the most commonly used:

When there is only one of something, either in existence or in a particular context. I'd like to speak with the manager.
When something is mentioned again. I've read three novels and two plays by Camus. What did you think of the plays?
When both listener and speaker know what is being referred to. Hurry up! The film starts in 5 minutes.
When talking about a specific aspect of something, where the noun is followed by of. We're studying the history of architecture.
To speak generally about certain groups of singular countable nouns. Interventions: The mobile phone is thought to pose a serious threat to health. Animal species: The wale is still hunted by some countries such as Japan.
With adjectives referring to general classes of people, ex: the homeless, the blind, the rich, ...
With superlatives. Who is the greatest footballer in the world.
With musical instruments. I'd love to learn to play the piano.
With types of transport which have a fixed timetable. Shall we get the bus or take a taxi?
With some countries, ex: the USA, the UK.
With oceans, montain ranges, deserts, rivers, etc. The Atlantic, the Pyrenees, ...
With some geographical areas. We're going to the mountains rather than the coast.


The indefinite article (a/an) is most commonly used:

When a singular countable noun is mentioned for the first time. A man went into a bar with a fish.
When referring to any one of several things. It's quiet in here. Shall I put a record on?
When talking about a person's job. My father is a nuclear scientist.
With some numbers, ex: a hundred and one dalmatians, a thousand people, ...
When it means "per" in some expressions, ex: twice a day, 50 miles an hour, ...


No article is used:

When referring to nouns in a general sense. Plural countable nouns. Do you think computers will replace teachers? Abstract nouns. We sang songs of love and peace. Other uncountable nouns. Alan won't eat cheese or meat.
With most streets, towns, cities and countries. I went to Bond Street when I was in London.
When a town's name is used with a building, ex: Luton Airport, Cambridge University.
In many common expressions, ex: to go home, to go to work/school/..., to be at home/..., to be in hospital/...