Jhn 16:20 Verily, verily, I say unto you, That ye shall weep and lament, but the world shall rejoice: and ye shall be sorrowful, but your sorrow shall be turned into joy.
Really, really, I tell you, you might weep and will sing requiems, but the world order will be rejoicing. On the other hand, you will be in pain, but still your pain will transform itself into delight.
My sense is that Christ uses the introduction "amen" and "amen, amen" to change the mood of the conversation, making it lighter. Why? In normal life, when people insist on their honesty, it actually creates a sense of dishonesty, like a used car salesperson calling himself "Honest Jim." It is funny for Christ, who always tells the truth, to use a statement insisting that he is really truly being honest now. This phrase is always used when the conversation gets too "heavy," as it has here. This phrase introduces a lighter take on things.
How does this verse lighten the mood? There are a number of subtle things.
First, Christ says they may cry (subjunctive verb) and they will sing (future) death songs. The "may" referring to crying when addressing a group of men is funny for a couple or reasons, but you either see why or you don't. The singing part refers to cantorial chanting or praying at a funeral service.
Next, the verb referring to the world rejoicing is middle passive with the sense that the world is making itself happy.
Finally, the last part is obvious, the contradiction about pain becoming joy.This last verb is also middle passive. The pain will transforms itself. This would have to be said lightly because it is non-intuitive. We normally say that the pain of death passes over time. You get over it. However, what Christ says here is entirely unexpected, at least to the apostles, what they are sorrowful about will be the basis for their joy.
A subtle change in verb forms.
λέγω (1st sg pres ind act "I say" is from legô (lego) means "pick up", "choose for oneself", "pick out," and "count," but it used to mean "recount", "tell over", "say", "speak", "teach", "mean", "boast of", "tell of", "recite," nominate," and "command."
καὶ "And" is from kai, which is the conjunction joining phrases and clauses, "and," or "but." After words implying sameness, "as" (the same opinion as you). Used in series, joins positive with negative "Not only...but also." Also used to give emphasis, "even", "also," and "just."
κόσμος "World" is from kosmos, which mean "order", "good order", "ruler", "world order", "universe," and "the world of men." Matthew uses it when Christ is talking about the order in the universe, specifically the order of the world of men, as it is designed to be.
λυπηθήσεσθε (2nd pl fut ind pass) "Shall be sorrowful" is from (lypeo), which means "to grieve", "to vex", "to cause pain", "to cause grief," and, in the passive, "to be grieved", "to be distressed," and "to be in pain."
εἰς "Into" is from eis (eis), which means "into (of place)," "up to (of time)", "until (of time)", "as much as (of measure or limit)", "as far as (of measure or limit)", "towards (to express relation)", "in regard to (to express relation)", "of an end or limit," and "for (of purpose or object)."