“A” and “AN” – are indefinite articles and indicate general reference.

                          – Indefinite articles are not used with mass nouns like jewelry.

                             (wrong: A jewelry is a sound investment)
                             (right: Jewelry is a sound investment)

                          – are singular in form and cannot be used with plural noun

“THE” – is a definite article and indicates specific reference.

             – it may be used with both singular and plural nouns.

Examples:
 An illustration will make your point more understandable.
The illustration he presented did not make the matter more understandable.

“A” – is used when the following noun begins with a consonant.

Examples:
- a book            - a towel
- a total failure   - a screaming child
- a football player - a wicked form of punishment

“A” – is used in front of vowels when their pronunciation sounds like you.

Examples:
- a European
- a unit
- a university

“AN” – is used with nouns that begin with a vowel (a, e, i, o, u).

Examples:
- an apple                  - an inquiry
- an indefinite answer      - an oval-shaped table
- an invitation tournament  - an inhumane form of punishment

“AN” – is used in front of a silent h because judging only by sound.

Examples:
- an hour
- an honor
- an herb

Note:

• If the noun is modified by an adjective, the choice between a and an depends
  on the initial sound of the adjective that immediately follows the article:
  

  Examples:
  - a broken egg         - an unusual problem- a European county (sounds like 'yer-o-pi-an, begins with

  consonant "y" sound)

   

• The indefinite articles are also used to indicate membership in a profession, nation, or religion.

Examples:
- I am a teacher.
- Patrick is an Irishman.
- Joe is a practicing Buddhist.

• Abstract nouns like “justice” and “independence” do not require indefinite articles.

wrong: They are looking for a justice.
right: They are looking for justice.

“THE” – is used with mass nouns for specific reference.

Example:
The jewelry was impounded by the police as evidence.

“THE” – is NOT used with non-countable nouns referring to something in a general    sense.

(no article) Coffee is a popular drink.
(no article) Japanese was his native language.
(no article) Intelligence is difficult to quantify.

“THE” – is used with non-countable nouns that are made more specific by a limiting modifying phrase or clause.

Examples: 
- The coffee in my cup is too hot to drink.
- The Japanese he speaks is often heard in the countryside.
- The intelligence of animals is undeniable.

"THE" - is used when noun refers to something unique.
Examples:
1. The white house
2. The theory of relativity
3. The 2001 national budget

Note:
Some geographical uses of "the". Do not use the before:
- names and countries (Italy, Mexico, Bolivia) except the Netherlands and 
  the US.
- names of cities, towns or states (Seoul, Manitoba, Miami).
- names of street (Washington Blvd., Main St.).
- names of lakes and bays (Lake Erie) except with a group of lakes like 
  the Great Lakes.
- names of mountains (Mount Everest) except with ranges of mountains like
  the Rockies or unusual names like Matterhorn.
- names of continents (Europe, Australia etc...).
- names of islands (Hundred Islands, Maui, Key West) except with Island
  chains like the Aleutians, the Hebrides, or the Canary Islands.

Do use "the" before:
- names of rivers, oceans and seas (the Nile, the Pacific).
- points on the globe (the Equator, the North Pole).
- geographical areas (the Middle East, the West).
- deserts, forests, gulfs, and peninsulas (the Sahara, the Persian Gulf, 
  the Black Forest, the Iberian Peninsula)