We use 'the', when there is only one of something.
We can use 'the' with the superlatives.

(ex) What is the highest mountain in the world?
(The highest mountain is only one in the world)
(ex) Your car is the most expensive car in our school.
(the superlative)

We can use 'the', when something (noun) is referred in previous sentence.

(ex) I have a computer, and the computer is made in U.S.A
(ex) I have a pencil with me, and the pencil costs $2.

Sometimes when nouns with qualifying phrases, you can use the nouns with 'the'.

(ex) The history of Chinese is interesting.
(ex) This is the pencil that I used the day before yesterday.

The superlative : the most beautiful, the best, the oldest, .....
The ordinal numbers : the first, the second, the third, the forth, .....
We use the before very, only, same.

(ex) She is the most beautiful women in our school.
(ex) It was the first time that he won in the race.
(ex) Your English book is the same as mine.

When we refer to all members of a group, we use the + adjective.

	the employed the dead the sick the homeless the young the rich the injured the old .....
	Example : The employed = people who have a job

(ex) The rich should pay more tax than the poor.
(ex) The young can do anything.

The + nationality is the people of that country.

(ex) The French love their country.
(ex) The Chinese have the great of wall.

a government office, a public institution

a ship, a train, an airplane

a river, a peninsula, a sea, a canal, mountains

a newspaper, a magazine

(ex) She usually reads the Time.
(ex) When he visited the White House, there were many people.