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. Albert Einstein was -select- a an the no article needed famous scientist.

Answer is "a"

2. Einstein was born in -select- a an the no article needed Germany in 1879.

Answer is "no article needed"

3. Einstein won -select- a an the no article needed Nobel Prize in Physics in 1921.

Answer is "the"

4. Einstein left his country and lived in -select- a an the no article needed States until he died in 1955.

Answer is "the"

5. Einstein is known for his theory of -select- a an the no article needed relativity.

Answer is "no article needed"

6. -select- A An The No article needed German physicist, who discovered X-rays in 1895, was named Wilhelm Roentgen.

Answer is "the"

7. Roentgen won -select- a an the no article needed 1901 Nobel Prize.

Answer is "a"

8. Mandela was born in -select- a an the no article needed South Africa.

Answer is "no article needed"

9. Mandela was -select- a an the no article needed first President elected in South Africa after Apartheid was revoked.

Answer is "the"

10. Mandela was imprisoned for -select- a an the no article needed nearly 30 years for his anti-apartheid activities.

Answer is "no article needed"

11. Mother Teresa was -select- a an the no article needed Roman Catholic nun.

Answer is "a"

12. Mother Teresa became famous for her hard work with -select- a an the no article needed poor.

Answer is "the"

13. She was -select- a an the no article needed founder of order of nuns called the Missionaries of Charity.

Answer is "the"

14. Mother Teresa lived in -select- a an the no article needed Calcutta, India.

Answer is "no article needed"

15. Mother Teresa received -select- a an the no article needed her Nobel Peace Prize in 1979.

Answer is "no article needed"