==> So there NO rules???
The rule was presented by sleipnir214 above. Use 'a' before consonant sounds and 'an' before vowell sounds.
Whether or not a leading 'h' is pronounced or silent is really a separte separate issue, but it does impact this thread. In those cases where the 'h' is pronounced, how strong is the 'h'? The stronger the 'h' pronunciation, the stronger the consonant sound, increasing the likelihood of using 'a'. Conversely, the weaker the 'h', the stronger the vowell sound becomes, therefore increasing the likelihood of using 'an'.
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