DEFINITE ARTICLE - THE

You use the when you have already mentioned the thing you are talking about.

For example:

"She's got two children; a girl and a boy. The girl's eight and the boy's fourteen."

We use the to talk about geographical points on the globe.

For example:

the North Pole, the equator

We use the to talk about rivers, oceans and seas

For example:

the Nile, the Pacific, the English channel

We also use the before certain nouns when we know there is only one of a particular thing.

For example:

the rain, the sun, the wind, the world, the earth, the White House etc..

However if you want to describe a particular instance of these you should use a/an.

For example:

"I could hear the wind." / "There's a cold wind blowing." "What are your plans for the future?" / "She has a promising future ahead of her." The is also used to say that a particular person or thing being mentioned is the best, most famous, etc. In this use, 'the' is usually given strong pronunciation whether or not it preceeds a vowel: For example: "Harry's Bar is the place to go." "You don't mean you met the Tony Blair, do you?" !Note - The doesn't mean all:- For example: "The books are expensive." = (Not all books are expensive, just the ones I'm talking about.) "Books are expensive." = (All books are expensive.)