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Use of “a” versus “an”

I have always been using a established.

The CPM is a established theory that explains.........

But when reading print material, I sometimes find an established being used:

The CPM is an established theory that explains.........

My English teacher taught me to apply a/an based on how the word sounded. Is that a/an established principle?

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marked as duplicate by RegDwigнt May 28 '11 at 16:34

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

    
Hi user8944 and welcome to the site. Please have a look at the "faq" tab under "Questions". Three out of the top ten questions right now deal with a vs. an. Have a look at the related questions linked at the right-hand side of this page, too. Thanks and welcome again. –  RegDwigнt May 28 '11 at 16:37
    
@RegDwight Cheers! By the way, I was constantly checking the Related tab as I was typing up my question, somehow that does not work because after I posted the question, the Related tab changed and came up with really meaningful "related" questions that were dealing with the same topic. I will raise this on the meta as I have seen this happening a lot of late on other SEs are well. –  user8944 May 28 '11 at 16:41
    
the key is using as many (appropriate) tags as you can think of. See here. –  RegDwigнt May 28 '11 at 16:46

2 Answers 2

up vote 1 down vote accepted

"An established" is correct. You use "an" before a word that starts with a vowel sound, as does "established."

BTW, this is based on how it is sounds not on how it is written. It is the reason why "a hotel" and "an hotel" can both be correct depending on the regional dialect, since in some dialects the "h" sound is dropped, and in some it is not dropped.

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Generally speaking use an before a word that starts with a vowel, and a before a word that starts with a consonant. So, in this case, an established.

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