Postby QuackingShoe » July 18th, 2009 7:48 am
Because that particle is written with the character that's pronounce 'ha'.
I know, I'm so banal, right?
But no, seriously, unless you want a history lesson, there's not much more to it. There are three particles (へ、を、は) that are written differently than they're read, due to historical reasons. They're read え、お、わ, respectively.
by and large, these are the only exceptions in modern Japanese. But いう happens to be read as ゆう, so la la la.