"I was taught, 55 years ago, an is used if the beginning letteris a vowel, vowel sound or an H." Today, we do not automatically use "an" in front of an "H", eventhough I just… did it there! For instance, I would say "She has ahome", not "She has an home". .
"An" or the long a sound (ay) is appropriate when a word beginswith a vowel SOUND. When a word begins with a consonant sound, thesound of "a" is a schwa, and "an" cannot be used. For example, theexpression "an hour" is correct because the initial sound of theword is a vowel sound. In regions of the English-speaking worldwhere the initial sound of "historic" is a short "i" sound, the useof "an" before "historic" would be correct, as in Cockney English.However, in most of the USA, the initial sound of "historic" is aconsonant sound, "h," and so it is correct to say "a historic day,"or "a historic moment." The original post also said 55 years ago "an is used if thebeginning letter is a vowel," but this is no longer as much of arule as it once was. We do not say "an university" because eventhough the word begins with a vowel, the sound of the "u" in thatword is the sound of a consonant, the same consonant in 'yak." I am 68 and a former English teacher who has taught in severaldifferent countries as well as the USA. A lot of the "rules" welearned decades ago were simplified by our teachers for us to learnmore easily. However, as we develop, we sometimes discover that therules are more complex and even have occasional exceptions. Onceexample of this is the "i" before "e" except after "c" rule--whichlater had added a secondary rule "or when sounded like "a" as inneighbor and weigh (or sleigh)." Even this new, improved rule hadother exceptions: science, weird, seize, either/neither, leisure,sheik, financier--as well as all comparatives and superlatives ofwords that end that end in a "c" followed by "y": e.g., fancier,fanciest. ( Full Answer )