THE is for specific and one-of-a-kind nouns.

A and AN are for general count nouns.

THE or nothing can be used with non-count nouns.

1.Use a with words that begin with a consonant sounds (b, c, d, f, g, h, j, k, l, m, n, p, q, r, s, t, v, w, x, y or z). There was a slice of pizza right here a minute ago. Where did it go?
The captain gave a whistle as he walked up to the deck.
2. Use an with words that begin with vowel sounds (a, e, i, o, u). Did you take an apple from this bowl?
It is an honor to accept this award.
3. a and an are the indefinite articles. They refer to something not specifically known to the listener. I saw an elephant this morning. (could be any elephant)
I met a clown on Washington Boulevard. (could be any clown)
4. Use a and an with something you have not talked before.
Use the with things you have already talked about.
She has two children, a girl and a boy. (first instance)
The girl's eight and the boy's fourteen. (subsequent instances)
5. Use the when you know that the listener understands what person/thing you are talking about. Did you lock the car?
Have you been to the doctor yet?
6. the is also used in the superlative and with things that are "the best". In this case, the is emphasized. Hardy's Grill is the place to go for ribs in town.
7. Use the with geographical points on Earth. the North Pole; the Equator
8. Use the with rivers, oceans and seas. the Nile; the Pacific; the English Channel
9. Use the with certain nouns when we know there is only one of a particular thing. the sun; the wind; the world; the earth; the White House
What are your plans for the future?
I could hear the wind.
10. Use the with places that have multiple states/areas. the UK (United Kingdom); the US (United States of America); the Irish Republic; the Netherlands; the Philippines; the British Isles
11. Use the with institutions and organizations with "of" or "for" She worked for the National Association for the Advancement of Colored People.
Greg works for the Office of Higher Education.
He's going to the University of Michigan.
He was thinking of going to Michigan State University.
12. Do not use an article with sports and individual lakes and mountains. However, use the before groups of lakes and mountains (mountain ranges). Tennis can be expensive.
My son plays football.
We visited Lake Michigan.
I have hiked Mount Fuji.
Have you ever been to the Great Lakes? (a group of lakes)
Have you ever been to the Rocky Mountains? (a mountain range)
13. Do not use an article with uncountable nouns when talking about them generally. Information is important to any organization.
Coffee can be good and bad for you. (any coffee)
14. Do not use an article with names of countries except when they indicate multiple areas or contain the words State, Kingdom, Republic, or Union. Italy, Mexico, Bolivia, England
15. Do not use an article when talking about things in general. People are worried about rising crime. (crime in general)

Rules - Ex.1 - Ex.2 - Ex.3 - Rules - Ex.4 - Ex.5 - Ex.6 - Ex.7 - Ex.8 - Ex.9 - Ex.10 - Ex.11 - Ex.12