English Translation
I am going to tell you the truth.
Why “σου” and not “σας”?
Either would be correct Greek, and both could be translated ‘I will tell you the truth’.
‘Σου᾽ is used here to indicate that you are speaking to one person. ‘Σας’ could be used to indicate that you are speaking to more than one person, or that you are speaking to one person more formally.