VOA慢速英语:Using the Right Article

Basic Uses of Articles in English

Indefinite Article (a or an)

Singular, non-specific nouns; I am a teacher; She has a dog.
   Use a with singular nouns beginning with a consonant sound: a user (sounds like 'yoo-zer,' i.e. begins with a consonant 'y' sound, so 'a' is used); a university; a unicycle
   an with nouns starting with silent "h": an hour
   a with nouns starting with a pronounced "h": a horse

Definite Article (the)

Specific plural or singular nouns; non-count nouns: I spilled the milk; 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

Zero Article (nothing)

Proper nouns such as Picasso, Paul, Ringo, John, George; names of places and institutions which consist of a proper name and another noun, e.g. Michigan State University, Trafalgar Square, Penn Station; names of days, months, seasons and holidays, e.g. on Sunday, in March, at Christmas; many geographical names, e.g. Europe, Italy, Tokyo, Lake Superior; certain prepositional phrases, e.g. at peace, by heart, by train/car.