Can someone explain why you may be "a" registered nurse but have "an" R.N. degree, be "a" medical doctor but have "an" M.D.? Why is it different to be "a" doctor of philosophy and have "a" Ph.D.?
First, welcome.
If I understand your question correctly, it has to do with why the form of the article a or an changes when a phrase is abbreviated. This is a very good question. While the answer is, in some ways, simple, there are some complications.
Importantly, for a and an, the choice has nothing to do with spelling and everything to do with sound. This holds true even if the words are never intended to be read aloud or spoken. If the sound following is a vowel sound, we choose an. If the sound following is a consonant sound, we choose a.
Since the letter H at the beginning of English words is sometimes pronounced as a consonant, and sometimes silent, we can demonstrate that the choice rests completely on the sound of the word, rather than on the spelling: a home; an honor; a horse; an hour. Like H, some words beginning with the letter U are pronounced with a consonantal /y/ sound at the beginning, while others are not: a uniform; an unknown; a unit; an underling.
Many consonant letter names start with vowel sounds (F, H, L, M, N, R, S, X). Conversely, one vowel name starts with a consonant sound (U). So abbreviations could change the form of the article from the full phrase:
a registered nurse / an RN
a medical doctor / an MD
a federal agent / an FBI agent
Here is where it can get complicated. Sometimes it is not clear if an abbreviation will be spelled out, or pronounced as a word:
an unidentified flying object / a UFO (Here it doesn't matter if you spell it /yoo-eff-oh/ or pronounce it /yoofoh/.)
But consider the case of the technology abbreviation SQL. It is sometimes spelled out /ess-kyoo-ell/, and sometimes pronounced /seequel/. You can tell which the author intends by looking at the article he or she chooses:
an SQL query
a SQL server
Likewise, the baseball abbreviation RBI is sometimes spelled out /ar-bee-eye/, and sometimes pronounced /ribee/.
an RBI
a RBI single (note that the photo caption has a RBI single, whereas the text of the article has an RBI double and an RBI single.)
My best advice is to trust your ear.
As usual, Glenn has the best answer, and I subscribe to most of it.
However, Glenn, as a lover of baseball myself, I cannot countenance anyone pronouncing "RBI" as "RIB-ee". I know it happens on sports shows (often on "Baseball Tonight", a show I watch religiously once the season begins), but it still bothers me. (Actually, another question might be why "Run Batted In" should be given the plural "RBIs", when the actual plural should be "Runs Batted In" — so "RsBI". Nevermind, I can see why that didn't catch on.)
Anyway, Glenn's explanation for the use of an/a is wonderful, so follow that. On a side note, though, I'd ask the literati: what is the proper nonspecific article for the word historic? Is it "a historic event" or "an historic event"? I say the proper article is a, but what do I know?
I also use a historic …, but smart people disagree. I can understand the disagreement on the basis of pronunciation. Some people pronounce the H while others don't, just as with an herb / a herb or an humble … / a humble … . (I pronounce the h of historic and humble, but not of herb.)
What's really odd about historic is that some people who pronounce the H still write and try to pronounce the article as an. I consider that practice an affectation.
[edited for format and some clarity]
I would put the extra "n" in front of a word starting with a vowel in the same category as the marvelous practice of adding a "r" to the ends of words that aren't spelt with one. It seems to me thatt the extra "r" is added when one word ends with a vowel and the next one starts with a vowel, and the "r" in the middle separates the words. Though not always...
JFK was famous for talking about the island nation of "Cuber".
"...buildup on the island of Cuba. Within the past week..." (No added "r")
but "...Soviet assistance to Cuber, and I quote, ..." (separating the a's)
"which has tu[r]ned Cuber against its friends"
I always assumed that those places with an extra "r" were where all the silent Yankee r's migrated.
("located in the U S S Ah" and "the west-n hemispheah") 🙂
Note that the ah is not dropped when a vowel follows a word ending in ah.
"acting therefore, in the ..."
(Or, Dr Who style, it got dropped as an ending ah sound, and then re-inserted to separate the adjacent vowels sounds ...)
But then there's doing the warsh, or withdraral ...