化蟲跟蹤 The Dragonfly Spy
那獃子虎急急的解了韁繩，拉出馬去。行者道：「沙僧，你且陪師父坐這裡，等老孫跟他去，看他往那裡放馬。」三藏道：「悟空，你看便去看他，但只不可只管嘲他了。」行者道：「我曉得。」這大聖走出廳房，搖身一變，變作個紅蜻蜓兒，飛出前門，趕上八戒。那呆子虎急急的解了缰绳，拉出马去。行者道：「沙僧，你且陪师父坐这里，等老孙跟他去，看他往那里放马。」三藏道：「悟空，你看便去看他，但只不可只管嘲他了。」行者道：「我晓得。」这大圣走出厅房，摇身一变，变作个红蜻蜓儿，飞出前门，赶上八戒。
逐句解 Line by line
The blockhead, in a blundering rush, undid the tether and led the horse out.
虎急急 is a headlong, blustering rush; 解了韁繩, untied the halter-rope. He is not even pretending to be casual about it.
Pilgrim said, "Wujing, you keep the Master company here a while; let old Monkey follow him and see where he goes to graze the horse."
陪師父坐, sit and keep the master company. 往那裡放馬 is where he goes to graze — the 那裡 of an indirect question. Wukong has read the situation before Bajie is out of the gate.
Tripitaka said, "Wukong, if you go to watch him, then watch — only don't keep mocking him so."
只管嘲他, keep on jeering at him. This is a well-judged instruction from a master who knows both his disciples: go and watch, but leave the poor creature some dignity. Wukong will obey the first half.
Pilgrim said, "I know."
曉得, I know, with the light de. He does know. That is not the same as agreeing.
The Great Sage walked out of the hall, gave his body a shake and a change, turned into a red dragonfly, flew out the front gate, and caught up with Bajie.
搖身一變, a shake of the body and a change — the standard formula for Wukong's transformations, and one of the phrases every Chinese child knows from this book. 紅蜻蜓兒 is a little red dragonfly, chosen because it is small enough to hover at a back gate unnoticed. 趕上, caught up with.
A short bridge from the disciples' hall to Bajie's private folly, and it moves fast. His blundering rush at the tether gives him away before he reaches the gate; his eagerness is legible to everybody in the room, and Wukong decides at once to follow and watch the comedy through to the end.
Tripitaka's parting instruction is nicely judged — go and watch him, but do not keep mocking him — and it is exactly the sort of order Wukong keeps to the letter while breaking its spirit in half. The pull between the master's forbearance and the disciple's appetite for mischief is one of the running notes of their whole relationship.
The red dragonfly is the mechanism of the episode's central irony. Bajie believes himself alone and unobserved with the widow; in fact every word and gesture has an audience. The stage is set for the covert half of the four saints' test, in which a man who thinks nobody is looking convicts himself out of his own mouth.