article

"I love to sail over the water" (some specific body of water) or "I love to sail over water" (any water).

"He spilled the milk all over the floor" (some specific milk, perhaps the milk you bought earlier that day) or "He spilled milk all over the floor" (any milk).

"A/an" can be used only with count nouns.

"I need a bottle of water."

"I need a new glass of milk."

Most of the time, you can't say, "She wants a water," unless you're implying, say, a bottle of water.

GEOGRAPHICAL USE OF THE

There are some specific rules for using the with geographical nouns.

Do not use the before:

names of most countries/territories: Italy, Mexico, Bolivia; however, the Netherlands, the Dominican Republic, the Philippines, the United States

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

OMISSION OF ARTICLES

Some common types of nouns that don't take an article are:

Names of languages and nationalities: Chinese, English, Spanish, Russian (unless you are referring to the population of the nation: "The Spanish are known for their warm hospitality.")

Names of sports: volleyball, hockey, baseball

Names of academic subjects: mathematics, biology, history, computer science