Some people say “thee”; some say “thuh”, many people say both! Here is the short answer for why and when we say them.
In the daily practice of speaking, American English speakers strive to stay as relaxed as possible, in part because of the large number of vowels and consonant formations English has. We cannot enunciate each and every sound because we must join them to each other. Instead we create patterns of stressed and unstressed sounds that together contribute to the “rhythm” of English.
One predictable pattern is created by stressing important words and de-stressing unimportant words, such as the word “the”. There is no rule that says we must pronounce “the” as [thee], or we must pronounce it [thuh]. Many American English speakers prefer [thuh] because it uses the schwa [?] sound, which is English’s most relaxed sound. It is the ultimate de-stresser, phonetically-speaking. Yet we occasionally hear people say “thee”. Why is that?
Here are some reasons:
When a speaker is trying to think of what they want to say next, and they need a little bit more time to collect their thoughts, they might say “…theee…..” and then continue on when they have their thoughts organized. We often hear TV personalities (talk show hosts, news announcers) do this. It saves them from saying “…thuuuhhhhhhh….”, or “the…uhh.…”, both of which sound rather thoughtless, and it also saves them from having several seconds of dead time, which is a bad thing on TV.
In this YouTube video, John Stewart gives himself an extra moment to think by stretching out his sound with a longer “thee” (but then he says “uh” anyway). If you continue listening to the video, you’ll hear him use “thuh” with regularity. Video time codes: [thee, 2:09] [thuh, 2:13, 2:19, 2:25….]
HOWEVER…
Sometimes it isn’t about stalling for time or filling a gap. It’s a different kind of choice. “the” is a grammatical function word which usually plays a very minor role in our message, for example, in a prepositional phrase (in the meantime), or even more simply, before a noun (the reason). In these cases, when “the” is merely part of the connective tissue of the language, its sound may be determined by several factors–the speaker’s need for speed, or a need to relax, or a desire to keep “like sounds” with “like sounds”, or a desire to tailor one’s liaisons (acoustic blending of words) for optimal listener ease. Let’s look at each of these reasons.
a. Need for speed: Well, it’s a fact that ”ee” takes longer to say than “uh”. Schwa is our shortest vowel. So if a speaker wants to talk fast, they’ll probably favor “thuh”.
b. Need to relax: it’s also a fact that ”uh” is easier to say than “ee”. Schwa is formed without tongue, lip, or jaw movement; it is merely air vibrating across our vocal chords, so if you want to stay loose, use schwa (uh).
c. Desire to stay with ‘like sounds’: Using [thee] sometimes makes it easier to pronounce subsequent words which begin with similar vowels, e.g. thee_easiest, thee_ugly, thee_earliest, thee_apple, the_acorn, the_issues, etc.
d. Desire to tailor one’s sound: People who talk a lot and talk fast, and still want to be understood by their listeners, may decide to adjust the sound of “the” to match the following nouns, thereby creating a smoother sound.
In this next YouTube video, Lee Howell, speaking at the 2008 World Economic Forum, uses both pronunciations of “the”. I’ve provided time codes for a short section in which he says [thuh] before consonant-initial words, and [thee] before vowel-initial words. [Note about 0:23: I consider H to be more like a voiceless vowel than a consonant; to me, it’s just air!]
[0:18 thuh vertical], [0:23 thee horizontal], [0:29 thee issues], [0:34 thuh conclusion], [0:40 thee innovation]
Well, this was as short an answer as I could give. It’s really about the dynamics of sound as English speakers employ it. Hope that answers more questions than it raises for you.