I believe "-eth" is a 3rd person ending (he/she/it), e.g. If thy header files fail should be "faileth".
"The third person sing. Present, ending in th, as taketh, bringeth &c. is now confined to sermons and grave discourse." - John Hunter, Text-book of English Grammar 1848
My favourite -eth, from the start of Moonlighting's Atomic Shakespeare episode:
Good morrow, sir. I'm Lucentio, come hither for to see fair Padua, pleasant garden of great Italy, to seek out and happily begin a course of learning and studies herein.
