There are only two articles in English: the and a (or an). The is a definite article and is generally used to refer to a specific noun. A is an indefinite article and is generally used to modify a specific noun. When the noun is not specific, you don't use an article. For example:
Americans like to swim in the ocean. (general)
The American likes to swim in the ocean. The Brit doesn't. (specific)
Reading books is the best way to increase your knowledge. (general, although I would strike the word "books" and just say "Reading")
Reading the text book is the best way to pass the class. (specific)
Reading a text book is the best way to pass a class. (specific)
Based on the context, whether something is general or specific is not always clear. In fact, the presence or absence of an article might convey critical meaning. For example:
I saw lions at the zoo today. (implies that at some point during the day, I cast my eyes for at least a second on some lions).
I saw the lions at the zoo today. (implies that I went to the lion exhibit).
Whether you use a or an is a separate to
pic of its own. Suffice it to say that it depends on the sound of the word the article precedes.
If only it were this easy! Unfortunately, when you refer to or modify a noun that represents a geographic place, you have to memorize when you do and when you don't use an article. And it quickly gets very confusing.
Instead of listing out the rules, I'm going to give an example that illustrates how confusing the rules are.
When you refer to a lake, bay, mountain, continent, island or street by its name, you don't use an article (note that rivers are not included). For example: Lake Hartwell; Narraganset Bay; Mount Reiner; and the Mississippi River.
When you refer to a group of lakes, mountains or islands, you do use an article. For example: the Great Lakes; the Blue Ridge Mountains; the U.S. Virgin Islands.