There are only three articles in English: a, an, and the. You can also use no article (-).
Articles are a type of determiner. Remember that determiners go before nouns. You can only use one determiner before each noun.
You can think of the articles as going from very general to very specific, in this order:
No article (very general)
A / An (general)
The (specific)
NOTE: A and An are the same thing. We use a when the next word starts with a consonant and an when the next word starts with a vowel.
No article (the noun in general):
Books are for reading. [Talking about books in general.]
A / An (any of the noun)
Read a book. [Talking about any book.]
The (specific one of the noun)
You should read the book Moby-Dick. [Talking about a specific book.]
Don’t use a / an with uncountable nouns.
Can I have a money?
I am breathing an air.
Use a / an when you talk about something the first time. Once it is understood, use the.
My dad just got a dog. The dog’s name is Tim.
Don’t use any article to talk about something very generally.
Air is needed to live.
Dogs bark.
This is also true for institutions (a large important organization that has a particular purpose, for example a university or bank).
We say:
I’m in school.
to talk about being a student, and:
I’m in a school.
to talk about being in the actual building.
The same is true for: prison, jail, church, hospital, and university.
Countries: if a country has a political title in its name we usually use the.
the United States of America
the United Kingdom
the Kingdom of Cambodia
Otherwise, use no article:
America
England
Cambodia
If the country is a group of islands, we also usually use the.
the Phillipines
the Bahamas
the Solomon Islands
There are exceptions to these rules, most notably:
the Netherlands
Indonesia
We don’t use the with any continent:
Asia
Africa
Antarctica
Australia
Europe
North America
South America
We don’t use the with most lakes and mountains.
Tonlé Sap
Superior
Victoria
Mt. Everest
Phnom Aural
But, we do use the with mountain ranges:
the Cardamom Mountains
the Himalayas
the Rocky Mountains
the Dâmrei Mountains
We also use the with rivers:
the Mekong River
the Bassac River
the Nile River
Seas:
the Caspian Sea
the Dead Sea
the Black Sea
Deserts:
the Sahara Desert
the Arabian Desert
the Antarctic Desert