How would Shakespeare have said "Thank you"? Can't decide if it is thee or thou, since it isn't really a sentence.
On a quick look through the concordance, it appears that Shakespeare rarely wrote Thank you and never Thank thee without a subject. He often wrote I thank you and we thank you (and forms such as to thank thee and shall thank thee); but for a shorter form without a subject he usually used Thanks.
Thank thou would be ungrammatical, unless it was followed by an object, and would then be a command: Thank thou the king!. But there do not seem to be any instances of thank thou in his writings.
From A Midsummer Night's Dream Act 5 Scene 1.
PYRAMUS Sweet Moon, I thank thee for thy sunny beams.
I thank thee, Moon, for shining now so bright.
For by thy gracious, golden, glittering gleams,
I trust to take of truest Thisbe sight.—
These are of course sentences.
As regards a phrase used in courteous acknowledgement, the OED only has examples for thank you. Entering thank thee simply gets you back to thank you. And it would seem that its use as such was perhaps not in common currency at the time of Shakespeare (died 1616).
The OED provides one example before his time - a thank yow
14.. Why I can't be a Nun 159 in Early Eng. Poems & Lives Saints (1862) 142 ‘Thanke yow, lady’, quod I than.
But strictly speaking that would appear to be a sentence, not simply a couteous phrase such as we use today - (I) thank you, lady
A further example one containing both you and thee would seem to be subject to the same objection.
1631 B. Jonson Divell is Asse iv. ii. 21 in Wks. II Eith. Thanke you good Madame... Tay. Thanke thee, good Eyther-side.
In short I think the answer to the question is that the simple use of thank you for e.g. someone passing you the salt, was probably not around in the sixteenth-century. In any event one could argue that what we say today is a sentence. One is in effect saying I thank you.
So to that extent the answer is probably I thank thee as in the above examples.
Firstly you should realise that the English language was in a state of flux during Shakespeare's time. You will find inconsistencies. Shakespeare's English was not Old English -- it was Early Modern English.
- "Thank you" as used these days is an abbreviation of "I thank you". Online Etymology Dictionary
- Plural form: The modern 'you' is used for both singular and plural. In Shakespeare's day there was a distinction. For example it would make no sense to say "I thank thee" to a group of people. Instead you would have to say "I thank ye" (familiar form) or "I thank you" (polite form).
MACBETH: I thank you, gentlemen. (polite) Macbeth Act I, scene III
KING HENRY VIII: My noble gossips ... I thank ye heartily; so shall this lady. (A king isn't obliged to be polite to his subjects, especially when he is insulting them!) King Henry VIII Act V, scene V
- Singular form: It is possible to find "I thank you" and "I thank thee" in Shakespeare when spoken to an individual. The explanation is that the plural is used as a sign of respect to an elder or superior. In modern English this respectful form is the only one to survive.
WS2 has given an example of "I thank thee".
CLAUDIO: I thank you, good friend Lucio. (Claudio doesn't have to be so polite to his friend. It's a choice. He is adding a note of respect.) Measure For Measure Act I, scene II
- "Thank thou" is possible for the reason stated by Colin Fine. It would not be a complete utterance and 'thou' would be the subject.
Like the cognate German-Dutch verb "danken", the English verb "thank" requires an object after it, except in making emphasis in imperative mood in Literary English:
Mr. Brown gave me some food and drinks yesterday. I was happy. I thanked him for helping me.
Thank thou thy parents for giving thee life!
(from a comment by Level River St.)
the word "thou" is used for emphasis and is optional as we know that sentences in imperative mood have one of these two understood subjects: you (as singular) or you (as plural), or the archaic version thou and ye respectively.
According to the Online Etymology Dictionary, "thanks" is a shortened form of "I give you thanks". The "you" in that phrase is the indirect object of the verb. Therefore, you should use the objective form "thee" rather than the subjective form "thou".
On Wikipedia there is an article which might help: https://en.wikipedia.org/wiki/Thou#Declension
In the article it states that 'thou' is the nominative form and 'thee' is the oblique (accusative?) form. I think that if you are giving thanks to a person then the oblique form is the correct form.
So my understanding would be that 'I thank thee' is the proper formulation.