Originally Posted by
aggelos
When selecting between 'a' and 'an' to put before a word, what counts is the sound that you hear first when you pronounce the word, not necessarily the first written letter of the word. So, say the word (aloud or in your mind) and decide if its first sound is a consonant sound or a vowel sound. If it's a consonant sound, put 'a'; if it's a vowel sound, put 'an'. And don't forget that there are some words in which the first letter you see is not pronounced.
As a little test, decide if you would put 'a' or 'an' before the following words:
book, evening, hour, honor, hat, union, URL, urn
I'll provide the correct answers in a new post, after some people (hopefully) have taken the test.