Aprender Ingles: Articles.

English has two types of articles: definite (the) and indefinite (a, an.) The use of these articles depends mainly on whether you are referring to any member of a group, or to a specific member of a group:

1. Indefinite Articles: a and an
A and an signal that the noun modified is indefinite, referring to any member of a group. These indefinite articles are used with singular nouns when the noun is general; the corresponding indefinite quantity word some is used for plural general nouns. The rule is:
• a + singular noun beginning with a consonant: a boy
• an + singular noun beginning with a vowel: an elephant
• a + singular noun beginning with a consonant sound: a user (sounds like ‘yoo-zer,’ i.e. begins with a consonant ‘y’ sound, so ‘a’ is used)
• some + plural noun: some girls
If the noun is modified by an adjective, the choice between a and an depends on the initial sound of the adjective that immedately follows the article:
• a broken egg
• an unusual problem
• a European country (sounds like ‘yer-o-pi-an,’ i.e. begins with consonant ‘y’ sound)
Note also that in English, the indefinite articles are used to indicate membership in a profession, nation, or religion.
• I am a teacher.
• Brian is an Irishman.
• Seiko is a practicing Buddhist.

2. Definite Article: the

The definite article is used before singular and plural nouns when the noun is particular or specific. The signals that the noun is definite, that it refers to a particular member of a group.
The is not used with noncountable 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 noncountable nouns that are made more specific by a limiting modifying phrase or clause:

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 variable but undeniable.

The is also used when a noun refers to something unique:

the White House
the theory of relativity
the 1999 federal budget
Geographical uses of the
 
Do not use the before:
• names of countries (Italy, Mexico, Bolivia) except the Netherlands and the US
• names of cities, towns, or states (Seoul, Manitoba, Miami)
• names of streets (Washington Blvd., Main St.)
• names of lakes and bays (Lake Titicaca, Lake Erie) except with a group of lakes like the Great Lakes
• names of mountains (Mount Everest, Mount Fuji) except with ranges of mountains like the Andes or the Rockies or unusual names like the Matterhorn
• names of continents (Asia, Europe)
• names of islands (Easter Island, 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)