If you're talking about something general, you need to use "a" or "an". Ex: "Pick a card." or "Take a seat." You have options. The only difference between "a" and "an" is that "a" is used before words that begin with a consonant. For example: "I picked a card from the deck." "An" is used before words that begin with vowels. For example: "An apple." If you're talking about something specific, you use "the." Ex: "Is this the card you picked?" or "I found the book I was looking for."

Also "a"/"an" is not used with plurals..

Count and Noncount Nouns The can be used with noncount nouns, or the article can be omitted entirely. "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

Definite Article: the The definite article is used before singular and plural nouns when the noun is specific or particular. The signals that the noun is definite, that it refers to a particular member of a group. For example: "The dog that bit me ran away." Here, we're talking about a specific dog, the dog that bit me. "I was happy to see the policeman who saved my cat!" Here, we're talking about a particular policeman. Even if we don't know the policeman's name, it's still a particular policeman because it is the one who saved the cat. "I saw the elephant at the zoo." Here, we're talking about a specific noun. Probably there is only one elephant at the zoo.