The indefinite articles “a” and “an” are pretty straightforward in everyday language. You put “an” before words beginning in vowels and words beginning with vowel sounds; everything else gets “a.” It’s a little more complicated with acronyms, however, since they aren’t actual words. What do you do then?
Simple: use your ears.
Whether or not the words abbreviated in the acronym are consonants or vowels, you should go with whatever the pronunciation of the acronym dictates. This can differ a lot between acronyms that should be pronounced about the same way, so use what you know to make the right call.
For example, the Food and Drug Administration’s acronym FDA begins with a soft “e” sound. The head-mounted display acronym, HMD, begins with a hard “a” sound. These are known as initialisms because they aren’t read as words, and they’ll both have “an” attached where “food” and “head” did not. The same goes for any acronym that begins with a letter making a vowel sound, whether or not it’s a vowel.
However, if the acronym is read as a word, you should go with however the pronunciation works. As another example, if you say FAQs (frequently asked questions) as “fax” instead of saying it by letter, use “a” to match the consonant sound. If you sound out each letter, use “an.” This will, of course, vary based on how you and others choose to pronounce the acronym.
Ultimately, don’t always go with what your eyes suggest; choose what sounds right.