Marta asked me an interesting question:
I wondered whether you could explain to me the difference in pronouncing the article 'the'. In some words like: the luggage, the monkey or the lion 'the' sounds as it is written 'thuh'. However, in other words like the economy, the elephant, the owl 'the' sounds like 'thee'. How would I know which pronounciation to use. Is there any pattern?
Her question contained the answer: Most native English speakers are more comfortable saying "thuh" before a word beginning with a consonant, and saying "thee" before a word beginning with a vowel. If we try to say "thuh economy," it sounds like a hiccup.
Sometimes I do hear people say "thee" before a consonant, but they're usually politicians trying to sound Very Serious: "We must protect thee principle of free trade."
By the way, the unstressed vowel sound is written phonetically as ǝ --technically known as a "schwa."