We use the indefinite article, a/an for:
Single countable nouns mentioned for the first time
– There’s a cat under the car.
– He’s a police officer.
We use the definite article, the for:
Previously mentioned nouns
– She came over to me and gave me a present and a card. The present was wrapped in beautiful paper.
When there’s only one of something
– I would really love to go to the Moon.
We use no article for:
Most languages, names, streets, towns, cities, countries (except the USA, the UK)
– I’m going to learn French and Italian at school.
– I’m going to invite James to the party.
– I live in Nelson Road, in Campbridge.
– The whole family have emigrated to Argentina.
– I’ve lived in the UK all my life.
Uncountable nouns, plural nouns and abstract nouns
– I’m going to buy bread and milk.
– It’s difficult to find accommodation around here.
– I like cats but I don’t like dogs.
– She gave us very good advice.