Here are the rules for when to use "A, An or The":

• a = indefinite article (not a specific object, one of a number of the same objects) with consonants
  She has a dog.
  I work in a factory.
• an = indefinite article (not a specific object, one of a number of the same objects) with vowels (a,e,i,o,u)
  Can I have an apple?
  She is an English teacher.
• the = definite article (a specific object that both the person speaking and the listener know)
  The car over there is fast.
  The teacher is very good, isn't he?
• The first time you speak of something use "a or an", the next time you repeat that object use "the".
  I live in a house. The house is quite old and has four bedrooms.
  I ate in a Chinese restaurant. The restaurant was very good.
• DO NOT use an article with countries, states, counties or provinces, lakes and mountains except when the country is a collection of states such as "The United States".
  He lives in Washington near Mount Rainier.
  They live in northern British Columbia.
• Use an article with bodies of water, oceans and seas -
  My country borders on the Pacific Ocean
• DO NOT use an article when you are speaking about things in general
  I like Russian tea.
  She likes reading books.
• DO NOT use an article when you are speaking about meals, places, and transport
  He has breakfast at home.
  I go to university.
  He comes to work by taxi.

Exercise

Complete the sentences with a suitable article.

1. I bought -select- a an the no article needed pair of pants and a shirt.

Answer is "a"

2. I saw -select- a an the no article needed car crash last week.

Answer is "a"

3. They are staying -select- a an the no article needed downtown.

Answer is "no article needed"

4. I think -select- a an the no article needed man is very unfriendly.

Answer is "the"

5. I don't like -select- a an the no article needed soccer.

Answer is "no article needed"

6. That is -select- a an the no article needed issue between you and the boss.

Answer is "an"

7. -select- a an the no article needed water is dirty. Don't drink it!

Answer is "the"

8. -select- a an the no article needed price of gas keeps rising.

Answer is "the"

9. John moved to -select- a an the no article needed San Francisco.

Answer is "no article needed"

10. Celine is -select- a an the no article needed French.

Answer is "no article needed"

11. -select- a an the no article needed amazing thing happened to me yesterday.

Answer is "an"

12. My friend doesn't eat -select- a an the no article needed red meat.

Answer is "no article needed"