¿When use the?

There are few rules to use "The"

• when we are thinking of one particular thing

for example:

• Tom sat down on the chair nearest the door.
• when it is clear in the situation which thing or person we mean.

For example:

• In a room we talk about the light, the floor, the ceiling
• when there is only one of something 

For example:

• What is the longest river in the world?
• we say: the sky, the sea, the ground, the country, the environment.
• (go to) the cinema, the theatre
• the radio, but television is without the.
• we use the before same (the same)