陰氣還重 The Yin Is Still Heavy
行者道：「或者是個軟柄槍；但這怪物還不會說話，想是還未歸人道，陰氣還重。只怕天明時陽氣勝，他必要走。但走時，一定趕上，不可放他。」八戒道：「正是，正是。」行者道：「或者是个软柄枪；但这怪物还不会说话，想是还未归人道，阴气还重。只怕天明时阳气胜，他必要走。但走时，一定赶上，不可放他。」八戒道：「正是，正是。」
逐句解 Line by line
Pilgrim said, "It may be a soft-shafted spear at that.
A concession — and the semicolon at the end of it does not license joining this to what follows, because the next clause changes the subject from the weapon to the creature and opens with a contrastive.
"But this creature still cannot speak. I take it that it has not yet come over into the human way, and the yin in it is still heavy.
還 is hái, "still," in all three of its appearances in this one segment. 歸人道 is to cross over into the human condition — the stage at which a creature under long cultivation acquires speech. Yin and yang are the paired principles of Chinese cosmology, yin being dark, cold, nocturnal and heavy; a creature that has not yet crossed over is still weighted to that side.
"I only fear that when day breaks and the yang prevails, he will certainly run."
The reasoning is cosmological rather than tactical: yin belongs to the night and has to give way when the sun comes up.
"But when he runs, we must be sure to come up with him. He is not to be let go."
一定 takes its ordinary tone slide before a falling tone.
Eight Rules said, "Just so, just so."
Seven characters, and the unit stands alone: it names its speaker and closes its own quotation, which is what keeps the shortest replies from folding into their neighbours.
Pilgrim concedes the spear and then explains the entire creature from the one fact Eight Rules keeps missing: it cannot talk. A thing under cultivation acquires speech when it crosses into the human way, so silence means it has not crossed, which means the yin in it is undiluted.
From that he derives a prediction. Yin belongs to night; day will break; the creature will have to run. It is the first piece of reasoning in the chapter that explains rather than dismisses, and it is built on the thing the disciples noticed from the courtyard — that it only ever parries.
The instruction after it is the one that matters for the rest of the chapter: when it runs, follow. Eight Rules agrees in four characters and the night ends there.