For the word 'year' to be preceded by 'an' it must sound like it's beginning with a vowel. The reason why it is a tad tricky is because of the difference in the way people pronounce it. Some people pronounce the word 'year' as 'ear' with 'y' silent, thereby wanting 'an' to precede and feeling discomfort with 'a'. e.g. More @Wikipedia
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