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.
- 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.
- 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.
- The second sentence correctly uses ‘the letter’ because we know ‘the letter’ is the same as ‘a letter’ mentioned in the first sentence.
- I received a letter. The letter is from a former classmate.
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