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 has been mentioned earlier. It is not about a noun that has not been mentioned beforehand or a noun that we are unaware of.
- 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.)
We use the in the following.
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, 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.