[ But in that moment, he realizes. Shen Qingqiu wouldn't do that. And if this boy doesn't believe he is Shen Qingqiu, but some kind of doppleganger, the situation might get complicated. He thinks about how the man acts, and also about people in his life who he thinks are a little similar to Shen Qingqiu. For a breath of time, he'd mistaken the man for Lan Wangji; but Shen Qingqiu is downright verbose compared to his old schoolmate. He thinks about it a little more. Yes, he'll be able to do this.

...

He still hasn't figured out what he wants to do. He snaps his fan open, to silence the boy. Finally, he speaks. ]

This teacher is disappointed. Every word that this disciple has spoken just now... is false. That this teacher will kill you, that this teacher does not trust you, that this teacher has never trusted you.

[ A little pause, as he considers what to do next. He himself would describe chapter and verse why the boy is wrong. Who kills their own disciple? Who tells somebody before killing them? He wouldn't have used the word "unless" to describe a worse fate, he was obviously going for something hopeful. But... Lan Wangji wouldn't do that. He would make a disciple think for himself. It's a better option, actually. People are more likely to believe something that they've thought through on their own. ]

Think for yourself why this must be the case. Think for yourself, and then tell this teacher your reasoning.