1) Why are there some exceptions, like ‘an MTR station’ and ‘a uniform’?
The most significant feature that triggers choices of ‘a’ or ‘an’ is based on a following sound, not a following letter.
British English: a herb (H is pronounced - /hɜːb/)
American English: an herb (H is silent - /ɜːrb/)
Specifically, judgement of vowel sounds does not depend on literal letters ‘a, e, i, o, u’ but on their IPA representation. (English vowels: /iː, ʊ, uː, e, ə, ɜː, ɔː, æ, ʌ, ɑː, ɒ, ɪə, eə, əʊ, aʊ, eɪ, aɪ, ɔɪ, ʊə/)
- MTR station – UK /em.tiː.ɑː steɪʃn/ US /em.tiː.ɑːr steɪʃn/
- Uniform – UK /juːnɪfɔːm/ US /juːnɪfɔːrm/
Although the letter ‘M’ is not a member of ‘a, e, i, o, u’ literally, the initial sound is pronounced as /e/, a vowel sound. Thus, ‘MTR station’ is paired with ‘an’ instead of ‘a’. Same for ‘uniform’, though the first letter is ‘u’, the initial sound /j/ is not a vowel. Hence, it is preceded by ‘a’, not ‘an’.