An article is used to describe the noun. In English, there are three articles — the, a, and an. ‘The’ is used to describe particular nouns; ‘a’ and ‘an’ are used to describe non-particular nouns.
The - definite article
a/an - indefinite article
‘The’ is used to mention something particular in a group. For example, in the sentence "Let's read the book," we are directed to think of a particular book, as compared to “Let’s read a book”, where the focus is more on the action of reading, than on a specific book.
"A/an" is used to mention a non-particular nouns. For example, "Let’s see a movie." Doesn’t mean that the speaker is interested in a particular movie; she might mean any movie.
We use ‘a’ when we are talking about non-specific singular nouns that start with a consonant sound. E.g. - a boy, a jar, a mike, a zoo, a dog.
We use ‘an’ when we are talking about non-specific nouns that start with vowel sound. E.g. - an elephant, an egg, an apple; an umbrella
Some words start with consonants but are pronounced as if they start with vowel sounds (the word ‘honest’ starts with ‘h’, but is pronounced as ‘onest’). These words will have ‘an’ before them as needed. For example - “He is an honest guy”.
Some words start with vowels but are pronounced as if they start with a consonant sound (the word ‘university’ starts with ‘u’ which is a vowel but is pronounced ‘yoo-niversity). In these cases, these words will start with ‘a’ when needed. For example – He will be applying for higher studies in a university.
These rules also apply on abbreviations and acronyms. An MRI scan, an MS Word document, etc.
‘The’ is used before particular and specific nouns. It is the only article that can be used for both singular and plural nouns. For example – “The boy who hit me ran away.” Here, we're talking about a specific boy.
In the sentence, “I saw an elephant at the zoo”, the speaker is not talking about any particular elephant, but he means a particular zoo.
‘The’ can be sometimes used with uncountable nouns to mention a specific case.
For example - “I threw the water as it was stored for too long.” In this sentence, the speaker clearly is talking about a specific batch/sample of water that was collected too long before and has gone stale now.
Similarly, in the sentence, “He spilled the milk” the usage of “the” makes it a specific sample of milk. The entire sentence could be “He spilled the milk you brought just now”
However, it’s very rare to find ‘a/an’ used with an uncountable noun (e.g. -I need a water)
‘The’ is not used before
Names of most countries/territories - Italy, Mexico, Bolivia. Exceptions - the Netherlands, the Dominican Republic, the Philippines, the United States
Names of lakes and bays - Lake Titicaca, Lake Erie. Exceptions - group of lakes, for example, the Great Lakes
Names of mountains - Mount Everest, Mount Fuji. Exceptions - ranges of mountains like the Andes, the Rockies
Names of islands - Easter Island, Maui, Key West. Exceptions - island chains like the Aleutians, the Hebrides, or the Canary Islands
Names of cities, towns, or states - Seoul, Manitoba, Miami
Names of streets - Washington Blvd., Main St.
Names of continents - Asia, Europe
‘The’ is used 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
There are some nouns that don’t use articles:
Names of languages and nationalities - Chinese, English, Spanish.
Names of sports - Volleyball, Hockey, Baseball
Names of academic subjects - Mathematics, Biology, History, etc.
Russian, etc. Exceptions - when you speak of them as a race - the Chinese are very hardworking.
When no specific noun has been mentioned, but a single noun is used to describe a category or practice. Also, if such things are mentioned in plurals. For example - (No article) Cats have bushy tails, No one can teach (no article) kids to behave.