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!!!"
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.