Examples:

• He watched a movie . The movie  was about the death of a ghost.

  (The movie mentioned in the second sentence is clear as it refers to the movie he watched.)

• I saw an old man with an eagle . The eagle  perched on his right shoulder.

  (The second sentence is correct: an eagle  becomes the eagle .)

• I received the letter . The letter is from  the former classmate .

  (The first sentence should use a letter , not the letter  as no one, except the receiver of the letter, knows which letter is referred to. The second sentence should use a former classmate , not the former classmate  because no one else knows which former classmate is referred to.)

Examples:

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 Eiffel 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, group 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, 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, the USSR.