A, an or The?

Some Korean students use 'a' or 'an' mixed with 'the.' Sometimes their teacher didn't tell the differences. But they're used in different meanings. Do you know the difference? Here is an information about the dfiference. So if you look the page below, you may know the differences.

Reference
http://www.englishclub.com/grammar/adjectives-determiners-the-a-an.htm