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".
vowel sound write say
a the apple thee apple
e the egg thee egg
i the ice-cream thee ice-cream
o the orange thee orange
u the ugli fruit thee ugli fruit
It is important to understand that it is what we say that matters, not what we write. It is the sound that matters, not the letter used in writing a word. So we use a long "thee" before a vowel sound, not necessarily before a vowel. Look at these cases:
we write with we say because
the house consonant h thuh house consonant sound
the hour consonant h thee our vowel sound
the university vowel u thuh youniversity consonant sound
the umbrella vowel u thee umbrella vowel sound
Emphatic the [thee] When we wish to place emphasis on a particular word, we can use "emphatic the" [thee], whether or not the word begins with a consonant or vowel sound. For example:
A: I saw the [thuh] President yesterday. B: What! The [thee] President of the United States? A: Yes, exactly!