賺入皮袋 Bagged at the Gate
那獃子不知好歹，就跟著他，徑回舊路，卻不向南洋海，隨赴火雲門，頃刻間到了門首。妖精進去道：「你休疑忌。他是我的故人，你進來。」獃子只得舉步入門。眾妖一齊吶喊，將八戒捉倒，裝於袋內，束緊了口繩，高吊在馱梁之上。那呆子不知好歹，就跟着他，径回旧路，却不向南洋海，随赴火云门，顷刻间到了门首。妖精进去道：「你休疑忌。他是我的故人，你进来。」呆子只得举步入门。众妖一齐呐喊，将八戒捉倒，装于袋内，束紧了口绳，高吊在驮梁之上。
逐句解 Line by line
The blockhead, not knowing good from ill, followed straight after him back the old road — but not toward the Southern Ocean; he went along instead to the Fire-Cloud gate, and in a moment reached the door.
不知好歹, not knowing good from bad, is a fixed phrase for someone with no judgement -- and the narrator uses it flatly, without softening. The devastating detail is the geography: 卻不向南洋海, not toward the Southern Ocean. Bajie is flying the wrong way and does not notice.
The demon went in and said: ‘Have no misgivings.
你休疑忌 -- have no misgivings. The demon says this at the threshold, which means Bajie has had some. Some part of him registered that this was wrong, and a kind word put it back to sleep.
He is an old friend of mine; come inside.’
And the last push: he is an old friend of mine, come in. Which happens to be true in the strangest way, since the cave-lord and the speaker are the same person.
The blockhead had no choice but to step through the gate.
只得 means had no choice but to -- and it is doing a lot of work. Nobody is forcing him. He has talked himself into a position where turning back would be rude in front of a Bodhisattva, and so he steps through.
The imps all raised a shout together, seized Eight Rules and threw him down, stuffed him into the bag, drew tight the mouth-cord, and hung him high upon the carrying-beam.
Fast and brutal after all that patient coaxing. A shout, seized, thrown down, stuffed in the bag, drawstring pulled tight -- the same cord the demon had replaced four passages ago because he did not trust the old one -- and hung up on the beam. And here the chapter's title lands: 木母被魔擒, the Wood-Mother is seized by the demon. 木母, Wood-Mother, is Bajie's name in the alchemical scheme that runs under the novel's surface, and this is the moment the second half of the couplet comes true.
The trap closes, and the passage is careful to show that Bajie was not overpowered. He was walked in. The narrator marks the point where the road should have turned south and did not; the demon offers one reassurance at the threshold, which tells you Bajie hesitated; and then he steps through the gate himself.
What takes him in is not stupidity so much as politeness. Turning round would have meant doubting a Bodhisattva to her face, and Wukong had specifically told him to bow and not look up. The instructions given to protect him are the ones that finish him.
This is where the chapter's title completes itself. The couplet reads 心猿遭火敗, 木母被魔擒 -- the Mind-Ape suffers defeat by fire, the Wood-Mother is seized by the demon. 心猿, Mind-Ape, is Wukong; 木母, Wood-Mother, is Bajie. Both halves have now happened, in order, and the chapter has delivered exactly what it advertised.