Sometimes the language rules we were taught as kids turn out to have been a bit oversimplified, especially if your adult career involves creative writing.
Remember learning to never start a sentence with “but” … and later finding out that you can certainly get away with it?
And that you use “a” before words that start with consonants and “an” before ones beginning with vowels … and then realizing you’d never actually say “a hour” or “an one-track mind”?
Even if you’re confident in your a/an distinctions, you’ll enjoy having Grammar Girl’s explanation of when to break the rules handy in case you’re ever wrongly accused of being, well, wrong.