低著頭 He Saw Nothing At All
行者道：「你只知鬧市叢中，你可曾看見那市上賣的是甚麼東西？」八戒道：「師父只教我低著頭，莫撞禍，實是不曾看見。」行者道：「你只知闹市丛中，你可曾看见那市上卖的是甚么东西？」八戒道：「师父只教我低着头，莫撞祸，实是不曾看见。」
逐句解 Line by line
Pilgrim said, "You know only about the thick of the market. Did you ever see what is on sale there?"
只知 grants the boast in order to dismiss it — he does not deny that Eight Rules frightens people, he changes what the market is for. 曾 reads céng, the adverb 'ever'. 可曾 is the one place in this chapter where the machine flips to zēng, which is a surname; the nine other occurrences of the character come out right unaided.
Eight Rules said, "The master told me only to keep my head down and not run into trouble. I truly did not see."
低著頭 occurs exactly twice in the chapter — in the street as the instruction, and here as the excuse. 著 is the weak zhe at both, against the zhuó of setting somebody to a task that the same character took a few passages back.
The trap closes in two lines. Eight Rules has spent the argument insisting the market is dangerous, and Pilgrim asks him what was in it — a question he cannot answer, because he obeyed the one instruction he was given.
The obedience is turned into the defect. Keeping his head down was the right thing to do and it is now the reason he missed everything, and he admits it with a plainness that makes the coming bait irresistible.
His last line is an honest sentence, and it is the last one he will be allowed before the food is described to him.