vowel前要用an, 同the 要讀 thi
e.g. an unhappy childhood
e.g. a university
而MTR係讀 em, ti:, ar
em 係 vowel, the 要讀 thi
Other examples:
F, H, L, N, R, S, X
Vowels: http://www.oupchina.com.hk/dict/phonetic/home.swf
the rule for pronouncing 'the' as 'de' is like this: not only for vowel sounds a, e, i, o, u, but also words starting with a letter with such a sound
e.g. mtr is pronounced as: 'em' tee ar, then the 'the' is read as 'de', because the sound following it begins with the sound 'e'