The A and AN articles are used with words with consonants and vowels respectively. For example: A car (the consonant C) An airplane (the vowel A) For this rule the letter Y is treated as a consonant: A yo-yo (Y as a consonant) Sometimes, there is an exception to using AN with certain vowels. Particularly the letter U. This is due to the similarity of the sound to the combination of Y and U (yoo): An uncle (the U has the short sound "uh" and is treated as a vowel) An uzi (the U has the long "oo" sound as is treated as a vowel) A uniform (the U has the long sound yoo and is treated like a consonant) Sometimes you will find the article AN used with the letter H: An honest mistake. This is because the letter H is silent. In British english the H in the herb (for example) is pronounced, where as in the USA, it is not, it would be pronounced "erb". The article you use will depend on how you pronounce it or want the reader to pronounce it: A herb. (the H is pronounced) An herb (the H is NOT pronounced) Either form would be correct although some auto-correct functions may disagree with you. Using AN can also force words that normally have the H pronounced to make the H go slient: A historian (the H is pronounced) An historian (the H is no longer pronounced) You will see a lot of debate on whether this is correct or not but regardless of whether it is proper grammar or not, the H becomes silent when you use the article AN. In certain accents, they forget what an H is altogether. They will distort words like: hard becomes 'ard helmet 'elmet hungry becomes 'ungry So, naturally, when they speak, they use the article AN: An 'ard day's work. He wore an 'elmet. An 'ungry wolf. They would be spoken this way, but never written this way, unless you are purposefully putting the accent into the mind of the reader. So, in regards to whether you should use an A or an AN, pronunciation trumps spelling.

Generally: 'a' is used before consonants, while 'an' is used before vowel sounds. Note that there are words that begin with a silent consonant (sometimes the letter h) and then you would use 'an' and not 'a' for those....

The meaning is the same. You use "an" when the word begins by a vowel and you use "a" when the word begins by a consonant or when the sound is a consonant sound. For example : an umbrella, a cat, but a union...

'a' is used before a noun that starts with a consonant (b, c, d...) and 'an' is used before a noun with a vowel (a, e, i o, u)