While we're all in class and without digressing too much, could someone please elaborate on the correct usage of the words 'a' and 'an'. I find myself perplexed sometimes as I try to get my get head around the proper usage, moreso of the word 'an'.

With some sentences it is quite easy and self explanatory, such as 'an X5 drove by' as opposed to 'a X5 drove by'. My understanding is that the former is correct. However, if one was to say 'My X5 needs a supercharger' or 'My X5 needs an supercharger', which one is correct?

It's no biggy, I'd just like to know the correct grammar

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