How do you choose between the two pronounciations of the words "a" and the word "the"?

Question

The word "a" can be pronounced in a schwa sound, such as "uh" or it can be pronounced with the long "A" sound, as in "A"... Similarly, the word "the" can be pronounced with a schwa ending, as in "thuh" or it can have a long "E" sound in the end, as in "th E" is there some sort of rule as when you use which pronounciation?

Answer 1

"thuh" is the usual pronounciation.

"thee" is used for either for emphasis "she is thee best singer"
or just because it's easier to say
"it's thee only one" - "thuh only one" doesn't run together so well.

Answer 2

This is a question of regional pronunciation. Both are essentially correct, but "the" is usually pronounced "thee" when the next word begins with a vowel. "Standard English," the kind spoken by anchorpersons on the media prefers "uh" (the schwa) for "a," but this doesn't mean that it's incorrect to pronounce the word using the long "a." You're the first person I've come across in YA who has used a linguistic term. Congratulations!