you don’t change a rule because people can’t be arsed pronouncing the H, if it starts a consonant You put “a”. The only exception to this rule (that springs to mind) is with acronyms, for example, “an SMS” because S starts with vowel sound: es
This sort of contradicts some of the research I did on the matter. http://icerfish.tumblr.com/post/645169460/a-vs-an
