There is more than one type of acronym. See the Wikipedia article here. Assuming you have in mind only the type of acronym called an "initialism," the choice of a or an depends upon the first sound pronounced when saying the initialism. If it is a consonant sound, use a. If it is a vowel sound, use an. Thus, it's "an MTR station" (m is pronounced "em") and "an HSBC branch" (h is pronounced "aitch").Hi all,
Should I use an or a before an acronym starting with M or H?
The band will be having a performance at an/a MTR (mass transit railway) station?
Where can I find an/a HSBC branch?
So, in a way, it could be 'a HSBC branch' or 'an HSBC branch' depending on whichever way the person writing it pronounces H?