“Yea, forsooth,” replied the bond-servant, staring with wide-open eyes at the scarlet letter, which, being a new-comer in the country, he had never before seen. “Yea, his honorable worship is within. But he hath a godly minister or two with him, and likewise a leech. Ye may not see his worship now.” This professor — who shall remain nameless; and he no longer teaches at my alma mater; he has moved on to another school in another state — this professor read yea (/jeı/) as yeah (/jɛə/). I’m sure he knew what the word meant, but he pronounced it incorrectly. He pronounced ye correctly, but that was small consolation to the ghost of Hawthorne, I’m sure. He (and others I’ve known) constantly confused the two words. And vice versa. In colloquial use, I see yeah misspelled yea all the time. It drives me absolutely nuts.
Can you imagine this? “Yeah, forsooth.” I would just about tear out my hair every time I heard it. He might as well have added, “dude.” This guy was a well-educated American college professor and a native speaker of English! The two words, yea and yeah, mean basically the same thing — yes — but there is a world of difference between them, starting with the pronunciation. I ask you, would it sound right to declaim, “Yeah, though I walk through the valley of the shadow of death […]”?
Or how about “Yay! Though I walk through the valley of the shadow of death, tra la la”? Maddening! The same confusion occurs with yay, the exclamation of delight, which I often see spelled yea. This is at least pronounced the same as yea, and at one time, its meaning may have been the same, but again, there is a world of difference now. I can understand the confusion to some degree. After all, the antonym of yea is not *nea, but nay. And all three, yea, yeah, and yay, may derive from the same source, Old English géa “yes”, but there is a reason we have three distinct forms today. I wish people would distinguish them appropriately.
I threw ye in for good measure (and because it occurs near yea in the Hawthorne quotation). This has two meanings: “you” and “the” — the latter, as in Ye Olde Fishe and Chippe Shoppe, comes from the loss of the thorn (þ) in the English alphabet and is really a corruption. No sign of affirmation in either of these words, and fortunately, I’ve seen fewer people confuse them.
Now hear ye: is anyone out there still unclear on the difference between these words, yea or nay? Nay? Yay! ;)
 Hawthorne, Nathaniel. The Scarlet Letter. Boston: Houghton Mifflin, 1878, pp. 129–30.