那是天鵝 That Is A Swan
行者指道：「那空中飛的不是？」八戒道：「那是一隻天鵝。」行者道：「正是老牛變的。」土地道：「既如此，卻怎麼好？」行者指道：「那空中飞的不是？」八戒道：「那是一只天鹅。」行者道：「正是老牛变的。」土地道：「既如此，却怎么好？」
逐句解 Line by line
Pilgrim pointed: "Is that not him, flying up there?"
The question is put as a negative, which in this idiom expects agreement: not "is that him?" but "that is him, surely". 空中 takes 中 at zhōng. (不 rises to bú before the falling tone of 是.)
Eight Rules said: "That is a swan."
Eight Rules answers the question that was asked and misses the one that was meant, which is the joke. (一 shifts to yì in 一隻, before the level tone of 隻.)
Pilgrim said: "It is the old bull, transformed."
Six characters correct him. The sentence closes on 的, so it identifies the swan as a thing produced by transformation rather than asserting that the bull changed — a distinction that matters for what follows, since the whole chase is about recognising a shape rather than a creature.
The local god said: "If that is so, what is to be done?"
The local god's second appeal for instructions in this chapter. He has been the practical intelligence of the party throughout — he blocked the retreat twice and refused the detour — and he asks rather than acts only now that the problem has become one of transformation, which is not his art.
Four short exchanges, and the middle one is the best line in the passage. Told to look at the thing flying overhead, Eight Rules identifies it correctly and completely misses the point: 那是一隻天鵝 — that is a swan. He is not wrong. He is answering a question about ornithology.
正是老牛變的 corrects him in six characters and states the chapter's remaining problem exactly. From here on nothing will be what it appears to be, and the only competence that matters is the ability to see through a shape.
既如此，卻怎麼好 is the local god asking for orders, and the request is a small concession. He has decided the party's course twice in this chapter, once by refusing the detour and once by blocking the cave mouth; now the contest has moved into an art he does not have, and he says so.