> With the definite article the, the weak form /ðə/ is generally used before consonants and /ði/ before vowels: /ðə ˈpeəʳ/ (the pear) and /ði ˈæpl/ (the apple). There is, however, a certain amount of variation: some speakers use /ðə/ throughout – and there are also those who have this as a strong form. (CGEL p.1613)

This is generally in agreement with what John Wells writes in the LPD:

> The English as a foreign language learner is advised to use ðə before a consonant sound (the boy, the house), ði before a vowel sound (the egg, the hour). Native speakers, however, sometimes ignore this distribution, in particular by using ðə before a vowel (which is in turn usually reinforced by a preceding ʔ), or by using ðiː in any environment, though especially before a hesitation pause. Furthermore, some speakers use stressed ðə as a strong form, rather than the usual ðiː.

On ELL, most students will likely be taught the traditional distinction between ðə and ði, and it's both simple and certainly a fine way for learners to pronounce things, so I don't see any problem with teaching it that way. We do have to acknowledge, though, that native speakers don't always follow that pattern.