The definite article (THE)

The definite article is generally used before a noun which has a clear reference. This means that it is clear who or what we are talking about because:

• the noun represents a unique thing or person in the context:
  

  The president has resigned. (there is only one president in the country)
  The earth is growing warmer and warmer. (there is only one planet Earth)

• the noun has been mentioned before:
  

  A man and a woman came in. The man was about 50 years old.

• a phrase or a clause after the noun makes it clear:
  

  Do you know the boy talking to Sally?
  This is the book that I told you about last week.
  The photo on the left was taken by my father.