It is a simple choice whether to use ‘a’ or ‘an’ as the indefinite article before words that begin with H.
It is purely based on pronunciation. If you aspirate (breathe) the H then use ‘a’, otherwise ‘an’.
When the first syllable is not accented as in ‘historic’ the initial H can be dropped, and in this case ‘an historic’ is fine. But please don’t use ‘an’ and aspirate the H. You will sound like an tool.
