The pronunciation of the word "the" depends on the first sound of the word that comes after it.
Normally, we pronounce 'the' with a short sound (like "thuh"). But when the comes before a vowel sound, we pronounce it as a long "thee".
For example: 'thuh' house vs. 'thee' apple.
Also when we want to emphasize for example:
A: I saw the [thuh] President yesterday.
B: What!! The [thee] President of the United States?
A: Yes, exactly!
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