Why is the article "a" sometimes pronounced with a short sound and sometimes pronounced with a long sound? And why is the word "the" sometimes pronounced "tha" and sometimes pronounced "thee"? Is there a rule for when each is used in English, or are they perfectly interchangable? Anyone? Maiku?

Using your symbols, generally, tha is used before consonant sounds, and generally, thee is used before vowel sounds. Thee is also often used to emphasize the status of something.

The long a is often used to emphasize that you are only speaking of one.

Of course, these aren't strict rules and they vary in different regions. I don't think there are any strict rules regarding this.

Methos' answer is right, basically. I'd just like to try to clarify a few points.

First, the pronunciations indicated for the (viz., with the vowel of thee when preceding a vowel sound, otherwise with the schwa of the second vowel in sofa) are very, very strictly obeyed by native speakers of English. They are not just pretty generally followed, but universally. I've never heard anyone but a non-speaker pronounce them differently.

Second, it is the following sound that determines, not the spelling, so that we get the with the vowel of thee directly before words like hour, but with the schwa directly before words union, which actually begin with the consonant sound of y.

Third, besides the very regular pronunciations of both the and a in connected speech, there are what are called citation forms, i.e., those speakers use when they are referring to the word itself, instead of using it. Thus, it would not be unusual to hear the pronounced like thee in sentences such as "The word the can be pronounced in different ways."

This so-called citation form often appears also as an emphatic, or contrastive form, as Methos has pointed out.

Finally, as a teacher, I've often noticed that students, when reading aloud from a text, will sometimes use the citation form inappropriately, even though they would never do so in connected speech.

Thank you, Methos and Maiku!

How about the article "a" with a long vowel vs. "a" as a short vowel (uh)?

Or glide vs. schwa.

I thought both Methos and I had already touched on the pronunciation of the indefinite article a.

In my previous reply, I ignored the fact that this word varies in a way which is exactly parallel to the. That is, it is pronounced as a schwa (your "uh") before consonants, but as an before vowels. The appearance of the word a with the vowel sound of day is restricted to the cases I mentioned above: (1) as the citation form; (2) in the reading aloud of certain not too sophisticated readers; and (3) as an emphatic or contrastive form, as Methos first mentioned.

I just re-stumbled across this. I know Maiku isn't around to reply, but I have to disagree with his statement that the rules I stated for pronouncing the are "very, very strictly obeyed by native speakers of English." In some Southern regions, the is pronounced tha even before many vowels.

"the is pronounced tha even before many vowels."

Not so much tha as th' ("Th' only way Ah'm agonna eat that is if you fix some grits with it.")