Definite Articles:

Use of “The”:

The is known as Definite Article.
It is used before singular and plural nouns.
It is used to refer to a particular member in the group.
Example:

1. The girl with the yellow shawl ran very fast.
   There are many girls but we are talking about a particular girl with the yellow shawl.
2. The black cat came inside the kitchen.
   There are many cats but we are talking about the black cat in particular.

Note:

“The” can be omitted when we are using uncountable nouns. “a”/”an” are used only in case of countable nouns.
Example:
I like to sail over water.

Do use the before:

Names of rivers, oceans and seas: the Pacific, Atlantic.
Points on the globe: the Equator.
Geographical areas: the West.
Deserts: the Sahara.

Do not use the before:

Names of countries: Mexico.
Names of lakes: Lake Superior .
Names of mountains: Mount Everest.