A vs. An

It's driving me crazy. Please PLEASE PLEASEEEEEEEEEEEEEEE! Learn grammar folks...

"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 choice of article is actually based upon the phonetic (sound) quality of the first letter in a word, not on the orthographic (written) representation of the letter. If the first letter makes a vowel-type sound, you use "an"; if the first letter would make a consonant-type sound, you use "a." 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'.

Source: http://owl.english.purdue.edu/owl/resource/591/01/