The definite article ‘the’ is used in the following cases.
1. When a singular noun represents a whole class.
The camel is a beast of burden. (Here the singular noun camel is used to refer to all the camels.)
Note that when a plural noun is used to talk about things in general, articles are usually omitted.
Camels are beasts of burden.
Computers are expensive.
More examples are given below:
The whale is a kind of mammal.
The rose smells sweet.
Note that the article ‘the’ is never used before the nouns ‘man’ and ‘woman’ when they represent the whole class.
Man is mortal. (NOT The man is mortal.)
2. While speaking of something or somebody already referred to
The boy who came to see me yesterday was my brother.
The story that he told us yesterday was very interesting.
3. While speaking of a particular person or thing
The poor beggar could get no alms.
4. When you refer to classics and holy books
Examples are: The Ramayana; The Mahabharata; The Iliad
Note that when the author’s name is mentioned with the book, the article is usually omitted.
Homer’s Iliad (NOT Homer’s the Iliad)
5. While referring to the names of journals and newspapers
The New York Times
The Wall Street Journal
6. When we refer to imaginary geographical lines
Examples are: The equator; the Tropic of Cancer; the Tropic of Capricorn; the latitude; the longitude