In reply to your last question, why we have 'a' or 'an' depending on whether the following noun starts with a vowel or a consonant, it's simply a matter of ease of pronunciation as well as how it sounds. Try eliminating "an" and using 'a' exclusively when speaking (recite or read something to hear yourself). When the following word starts with an vowel (or an 'h') there has to be a very slight hesitation between the two and it sounds gulping or breathy. That's why the 'n' is inserted (and creates 'an'); it makes the words flow smoothly without the hesitation or huffing sound you otherwise get, like when we use 'a' before an aspirated 'h'. ('An' was often used before 'h' in Elizabethan English -- see the Biblical verse referring to "an house not made with hands", but the practice was later dropped.) So its not a matter of grammar, syntax, derivation or philology -- just a practical matter of euphony.
