1. Definite article: the

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.

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.)

We use the in the following.

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 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.