I've noticed that there seems to be some confusion with when to use "a" and "an".  The rules are actually very simple, so I will state them and also give a site-link to as a reference.  Basically, "a" is used when the word following has a consonant SOUND, and "an" is used when the word following has a vowel SOUND.  The most common problem seems to be with the silent "H" at the beginning of many words.  I see a lot of "a hour" which is incorrect. 

Another issue is acronyms.  Again, it all depends on the sound of the way you say the first letter of the acronym.  "A FBI agent" is incorrect, as the letter "F" has a vowel sound when spoken. 

Also, a hard "H" would usually have an "a" preceding it. But as noted on the website I am listing, there is an odd situation with certain non-silent "H" words, where an "an" is acceptable, but the use is somewhat archaic.  "An historic event" is acceptable, but so is "A historic event" which I believe sounds better, but my opinion doesn't matter on this subject. 

To sum up, when using "a" or "an", you simply need to ask yourself if the word has a vowel sound at the beginning, or a consonant sound.  Basically just use what sounds right, without worrying about what the first letter of the word is. I hope this helps. 

Website a vs an