Snape has a time-turner.
Draco wonders where he got it, how it is that he manages to have such an item without the Ministry or someone figuring it out, what he uses it for. Draco doesn't ask any of these things.
He's never seen Snape's actually use it before, but somehow, Draco knows he has.
The question Draco most wishes he could ask--the question he never, never will--is, "What would you use it for, if you could use it for anything?"
Draco knows that there are limits to a time-turner's powers, that even if a person could go back to any time in history the risks would be more than the benefits could possibly be worth.
He also knows the answer to the question he will never, never ask. Draco saw Snape's face right before he used The Curse. (Draco still can't think the words. He needs to, he needs to, but he's tried and it's no use.) He wasn't supposed to be looking at Snape (well, no, he was supposed to be killing) but he had and Draco's quite sure he will never erase what he saw.
He imagines it looked like something else to everyone else there, everyone who wasn't thinking, no, I can't, no, help me, Snape everyone who wasn't at a moment of utter, absolute choice and would not have recognized that moment being worn on another's face. Draco saw, though.
He knows how many turns Snape would make, if only he could. He knows to the minute.
Draco himself would murmur a putesco and painstakingly count out the 196 days that have passed since Snape pulled him outside that stupid, meaningless party and said "I am trying to help you."
His wrist would no doubt be sore after each tiny manipulation, and he would still know all he knows now. But he would be able to say, "Please, please help," and not mean anything about the bloody cabinet. Nothing about Voldemort and glory and all those things that Draco had been so sure meant something.
Draco does not take the time-turner, however. He does not speak the word putesco, not even in the bleak of dawn, while Snape is out wherever Snape goes to, and nobody is around to hear him.
One morning, when Snape reappears from wherever he has disappeared to, Draco says, "I'm sorry."
Snape nods like he understands and Draco wants to ask for which crime Snape considers this apology to be given.
He stays silent.