I know thou, thee, thy, thine, but I'm not sure if thee is correct in
the sentence "Know thee, whereof thou speak'st" with the meaning "Be
sure you know what you are talking about"
Compare, singular:-
I - me - my - mine
thou - thee - thy - thine
plural:-
we - us - our - ours
you - you - your - yours
"ye" is a variant of "you". Some speakers say "ouren" and "youren"
instead of "ours" and "yours".
The first person singular is rarely used in modern English; so rarely
that when it is, the verb often takes an archaic "...[e]st" suffix
in the 'vocative' or 'interrogative' cases.
Some regional dialects have a similar pattern of usage to the French
"vous/tu" based on relative status or familial connection or affection.
Some people use the first person singular rigorously when addressing
only one person, for philosophical or logical reasons. "Quakers" and
some other religious groups are the best known examples of this; the
popular Bible translation originating in the early 17th century and
known as "The Authorised Version" (AV) or "King James Version" (KJV)
also does this, mostly.
So your "know thee whereof thou speakest" is essentially 'correct', the
verb 'to know' being used in the 'vocative' case and 'to speak' being
'nominative'. This is one of the relics of the time when English was
more fully 'inflected'.
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-- ^^^^^^^^^^
-- Whiskers
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