Where we have to pronounce the article the (dee) and the (da) before a vowel?

[Marmel]

English Grammar Tests, Elementary Level

ESL/EFL Test #98 "Articles in English Grammar", question 10

......... President of the United States will be visiting Australia next week.

(a) A
(b) An
(c) The

English Grammar Tests, Elementary Level

ESL/EFL Test #98 "Articles in English Grammar", answer 10

The President of the United States will be visiting Australia next week.

Correct answer: (c) The
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I would like to know where we have to pronounce the article the (dee) and the (da) before a vowel.

Marmel

[Mister Micawber]

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You do not 'have to' pronounce 'the' either way; it is not a rule or a requirement. It is just the physics of clear language production. It generally sounds like /dhi:/ when it is unstressed before a vowel sound, that is all.
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Native English teacher at Mister Micawber's