a , an , and the

There are only three articles in English: a, an and the. There are two types of articles indefinite 'a' and 'an' or definite 'the'. You also need to know when not to use an article.

Indefinite articles - a and an

A and an are the indefinite articles. They refer to something not specifically known to the person you are communicating with.
A and an are used before nouns that introduce something or someone you have not mentioned before
"I saw an elephant this morning."
"I ate a banana for lunch."
A and an are also used when talking about your profession:-
"I am an English teacher."
"I am a builder."

You use a when the noun you are referring to begins with a consonant (b, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, w, x, y or z), for example, "a city", "a factory", and "a hotel".
You use an when the noun you are referring to begins with a vowel (a, e, i, o, u)

Definite Article - the

You use the when you know that the listener knows or can work out what particular person/thing you are talking about.
"The apple you ate was rotten."
"Did you lock the car?"
You should also use the when you have already mentioned the thing you are talking about.
"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.
"the North Pole, the equator"
We use the to talk about rivers, oceans and seas
"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.
"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.
"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:
"Harry's Bar is the place to go."
"You don't mean you met the Tony Blair, do you?"