Regarding A vs An:
"A" goes before all words that begin with consonants.
a cat
a dog
a purple onion
a buffalo
a big apple
with one exception: Use an before unsounded h.
an honorable peace
an honest error
"An" goes before all words that begin with vowels:
an apricot
an egg
an Indian
an orbit
an uprising
with two exceptions: When u makes the same sound as the y in you, or o makes the same sound as w in won, then a is used.
a union
a united front
a unicorn
a used napkin
a U.S. ship
a one-legged man
Note: The exceptions for the articles are based upon the orthographic or written representation of the word-initial letter not the phonetic or sound quality of the letter. So, if you consider the rule from a phonetic perspective, there aren't any exceptions. Since the 'h' hasn't any phonetic representation, no audible sound, in the first exception, the sound that follows the article is a vowel; consequently, 'an' is used. In the second exception, the word-initial 'y' sound (unicorn) is actually a glide [j] phonetically, which has consonantal properties; consequently, it is treated as a consonant, requiring 'a'.
