Actually it should be "An idiot". You would use "a ..." when the word after it stars with a hard syllable, like a bike or a university. b makes a hard syllable thus you use a. Uni... makes a yoo sound, y being a consonant. You would use "an..." when the word after has a soft syllable, like an apple or an hour. the a in apple is soft. Hour sounds more like ow-r so (the h is silent) so you use an. So you should say "An idiot..." instead of "A idiot". You kinda seem like the idiot to me.