Because it is difficult to pronounce the letter (a ) before other letters such as ( a ,e,i,o, and u,) and the rule was put for this reason ,so we use it when we do not have the problem with words ..such as a university- a european country .and the same thing is used with an we say an hour -an honst man and so on . I think that our friend who explained this subject in Arabic is totally right .
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