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Would you please tell me your ideas about the meaning of this sentence: "You shall always tell the truth now." I'm also not sure about the tense and grammar of it.

Here is how I interpret it:
It is a kind of threat, which means "Soon, I am going to do something [horrible] to you which will make you tell the truth forever!!!"
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That much meaning cannot be extracted from the sentence, but you shall (shall in the 2nd person) expresses strong volition by the speaker-- a sort of command, threat or exercise of willpower.
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Thanks for your answer Mister Micawber. But I have two more questions:
1) You talked about shall in the 2nd person. What if the aforementioned sentence began like this:"I shall always..."?
2) What is your idea about the tense of this sentence? It is in present tense, right? I think we can replace "shall" with "will" (in British English I mean), am I right?
i think i would agree too. from the entire context its seems to me that the sentence has to do with a 'NOW' threat that will change your attitude towards telling the truth. from the term SHALL, it speaks for will when refering to the present only and therefore fits for a replacement of tenses.