A Ren Faire Speak How-To
Will and shall change to wilt and shalt when thou is the subject, but remain unchanged for other subjects.(Lesson L(50) from Illustrated Lessons in Our Language by G.P Quackenbos, 1882. Available on CD at Lady's Maid Books).
The remaining auxiliaries, also, change only when thou is the subject. But thou is not used in ordinary discourse. Except in solemn or poetical style, we use you, whether addressing one person or more; these auxiliaries, therefore are seldom changed.
The forms of the remaining auxiliaries required with thou, in solemn or poetical style, end in st: --
MUST - thou must (no change).
HAD - thou hadst.
CAN - thou canst.
MAY - thou mayst (mayest).
MIGHT - thou mightst (mightest).
COULD - thou couldst (couldest).
WOULD - thou wouldst (wouldest).
SHOULD - thou shouldst (shouldest).
...Will and shall may not be used at pleasure, the one for the other. To express simply what is about to take place, shall is used with I and we; will is used with all other subjects.
...Will used with I or we, and shall with other subjects imply determination as well as futurity.
On the topic of will and shall, The Common School Question Book (by Asa Craig, 1878) has this to say about them:
116. When is it proper to use shall and should?So, there is a difference between shall and will. It's just that very few of us know it anymore.
Answer: When required to express a duty, command, determination, resolve; and in future propositions when the subject is of the first person and no reference is made to the will of the subject.
117. When is it proper to use will and would?
Answer: When the expression is of willingness, inclination, or in future, propositions when the subject is of the second or third person, and no compulsion required.