Liland, just one problem there. It is thee IS as thee is by definition singular.
As Jessica's mum I have been using plain talk 25 years longer than she has but don't know much more about when the nominative/subjective went out of use. However the plural IS you ( so my kids always know if I am talking to one or more of them) When I was in school I got much grief for our family's use of "archaic language". Jessica's friends all think it is wonderful, and often try to use it--at least around here!
I HAVE been chided about the fact that since we tend to use plain talk only among family and close friends, that it is now the opposite of the way the Quakers originally meant it, being now exclusive instead of inclusive. I also know that I caught merry hell from my Dad when I used it with my boyfriend in HS.
Merry Christmas All!