"A" or "An"?

I've always struggled with this rule when I was learning english as a child. Which has been a long time now. haha. I hope this rule helps you too.
Click HERE for the link.
A or An Rule
"A" goes before all words that begin with consonants.
a cat , a dog, a purple onion

"An" goes before all words that begin with vowels:
an apricot, an egg, an Indian

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.
Examples: a union, a united front, a unicorn, a used napkin, a U.S. ship, a one-legged man