Articles specify whether nouns are definite or indefinite. The definite article "the" is used for specific nouns known to both speaker and listener, unique objects ("the sun"), superlatives ("the best"), and ordinal numbers ("the first"). With geographical nouns, "the" appears before oceans, seas, rivers, mountain ranges, and groups of islands ("the Pacific," "the Nile"), but not before individual mountains, lakes, cities, or continents.
The indefinite articles "a" and "an" refer to one unspecified item. The choice depends on sound, not spelling: "a" precedes consonant sounds ("a book," "a university" because of the "yu" sound), while "an" precedes vowel sounds ("an apple," "an hour" with silent "h"). This pronunciation rule applies to abbreviations as well ("an MBA," "a UNESCO site"). Jobs typically require "a/an" ("She is a doctor"), unless specifying a particular person ("the doctor who treated me").
The zero article (no article) is used with uncountable or plural nouns in general statements ("Life is short") and with most proper nouns ("I live in Paris"). Countable and uncountable nouns follow different quantifier rules: "fewer" pairs with countable nouns ("fewer books"), "less" with uncountable nouns ("less water"), "many" with countable nouns ("many friends"), and "much" with uncountable nouns ("much time"). "A few" and "a little" both mean "some" but apply to countable and uncountable nouns respectively.