The is the definite article as it identifies a definite or particular noun that we know of. It is not about a noun that has not been mentioned beforehand or a noun that we are unaware of.

EXAMPLE: I received a letter. The letter is from a former classmate.(CORRECT)
NOT: I received the letter. The letter is from the former classmate.
First sentence: I received the letter. – Which letter is referred to?
Second sentence: The letter is from the former classmate. – We know 'The letter' as it refers to the letter I received (in the first sentence). / the former classmate – There are many former classmates, which one?)
This shows how easily we can confuse the reader if we do not exercise care in using the.

The is used:
1. when the same thing or person is mentioned again, that is, a particular thing or person.

Example: I bought an orange. The orange is sweet.

2. when there is only one such thing.

Example: the earth, the sun, the moon.

3. before the names of famous buildings, etc.

Example: The Eiffel Tower, The Great Wall of China.

4. when a singular noun is used to point out a whole class, race, group, etc.

Example: The bear is a strong animal.

5. before the special names of a rivers, seas, oceans, mountain ranges, groups of islands, certain organizations, political parties, and countries such as the U.S.A., the U.K., the U.S.S.R. and the U.A.R.

Example: The Nile, The Dead Sea, The Pacific Ocean, The Himalayas, The United Nations, The Republican Party, etc.

6. before the names of holy or important books.

Example: The Koran, The Bible.

7. before an adjective when the noun is understood.

Example: The poor need help.
Example: No one seems to care for the homeless in the city.

More examples:

• This is the boy you are looking for. (This means this is the particular boy you are looking for, not any other boy.)
• That is the cinema I went to yesterday. (I remember and am sure that is the cinema I went to and not another cinema.)
• While they were there, they visited the science museum. (They visited the particular museum which is the only one in the country.)