"How to use the word "the?" That is correct. Well, this word is basic; it's elemental. At the same time, it's usage can be complexed. "The" is a definite article. In explanation, it is a defining addition to a word that yeah, does that, defines it. "The" before a word makes that word appear to be referring to a specific thing. "The dog" That phrase is referring to a specific dog, ONE dog that has a specific kind of traits. "A dog" That phrase is referring to any dog. It is not specific, but it is still one dog. When to use "the?" Use "the" when you want to specify what you are talking about. Also use "the" when you are claiming a title of something. Example "the President of the United States." There are many more places to use it, and sometimes they can't be explained in rule, they just sound better with or without "the." You just need to practice a lot!

http://www.italki.com/discussion/65491