The, the definite article, is one of the most common, if not the most common, words in English. The identifies a definite or particular noun that we know of because it has been mentioned. It is not about a noun that has not been mentioned beforehand or a noun that we are unaware of.

Examples :

  • He watched a movie. The movie was about the death of a ghost. (We are clear about which movie ‘the movie’ mentioned in the second sentence refers to.)
  • I saw an old man with an eagle. The eagle was perching on his right shoulder.
  • I received the letter. The letter is from the former classmate.
 
  1. The first sentence has to use ‘a letter’ not ‘the letter’ as no one, except the receiver of the letter, knows which letter is referred to.
  2. The second sentence correctly uses ‘the letter’ because we know ‘the letter’ is the same as ‘a letter’ mentioned in the first sentence.
However, we still don’t know which former classmate when it refers to ‘the former classmate’. This is not correct, so the sentence should be rewritten correctly as follow:
  • I received a letter. The letter is from a former classmate.
 
We use the:

when there is only one such person, place or thing

  • the Pope, the President of the United States, the North Pole, the earth, the sky

before names of famous buildings, etc

  • the Eiffer Tower, the Great Wall of China

before a singular noun that refers to a whole class or group of people or things

  • the middle class, the homeless, the Canadians, the Hindus

before the special names of rivers, seas, oceans, mountain ranges, groups of islands.

  • the Nile, the Dead Sea, the Pacific Ocean, the Himalayas

before certain organizations, political parties, and countries

  • the United Nations, the Republican Party, the USSR, and the UAR

before nouns such as places which we know of

  • We arrived early at the ferry terminal for our trip to the island.
  • We went to the cinema, after which we went to the stadium for a football match.

before abbreviations and initials of countries

  • the BBC (the British Broadcasting Corporation), the EEC (the European Economic Community).
  • the UAR., the UK, the USA and the USSR