Articles (a, an, the) are part of a category of words known as noun markers or determiners. Noun markers identify the nouns that follow them. Besides articles, noun markers include possessive nouns (Elena’s, child’s); possessive pronouns/adjectives (my, your, their); demonstrative pronouns/adjectives (this, that); quantifiers (all, few, neither, some); and numbers (one, twenty-six).
Use the with most nouns that the reader can identify specifically. Usually the identity will be clear to the reader for one of the following reasons.
The noun has been previously mentioned.
A phrase or clause following the noun restricts its identity.
A superlative adjective such as best or most intelligent makes the noun’s identity specific. (See also 27d.)
The noun describes a unique person, place, or thing.
The context or situation makes the noun’s identity clear.
The noun is singular and refers to a scientific class or category of items (most often animals, musical instruments, or inventions).
Use a or an with singular count nouns that refer to one unspecific item (not a whole category). Count nouns refer to persons, places, things, or ideas that can be counted: one girl, two girls; one city, three cities; one goose, four geese.
Do not use a or an with noncount nouns. Noncount nouns refer to things or abstract ideas that cannot be counted or made plural: salt, silver, air, furniture, patience, knowledge. (See Commonly used noncount nouns.)
To express an approximate amount of a noncount noun, use a quantifier such as some or more: some water, enough coffee, less violence. Do not use articles with nouns that refer to all of something or something in general.
NOTE: A few noncount nouns (such as love) can also be used as count nouns: He had two loves: music and archery.
Do not use articles with most singular proper nouns: Prime Minister Trudeau, Jamaica, Lake Huron, Ivy Street, Mount Everest. Use the with most plural proper nouns: the McGregors, the Bahamas, the Finger Lakes, the United States. Also use the with large regions, oceans, rivers, and mountain ranges: the Sahara, the Indian Ocean, the Amazon River, the Rocky Mountains. There are, however, many exceptions, especially with geographic names. Note exceptions when you encounter them or consult a native speaker or an ESL dictionary.