Now the fiend had sent Tripitaka out of the cave, and Sha Monk came forward to ask, "Master, you are out — but where has our elder brother got to?"

The chapter opens in the middle of a rescue that has already half happened. Pilgrim swallowed himself into the fiend at the end of the last chapter and has been bargaining from inside her ever since; the fiend has just handed Tripitaka out of the door to buy herself relief. 著 is zhuó, the little word that marks who did it — Tripitaka was sent out, and the fiend is the one who sent him. Ming vernacular used this character the way modern Chinese uses 被, and the novel leans on it constantly.

Bajie said, "He has a plan of his own. He will have traded himself for the master, depend on it."

貼換 is to swap one thing in for another, the way you would trade a hostage. Bajie is guessing, and his guess is wrong in a way that will matter: he assumes Pilgrim has bought the master's freedom by staying behind as security. Hold that thought — the misreading is what makes him lose his temper seven passages from now.

Tripitaka pointed at the fiend with his hand and said, "Your elder brother is in her belly."

A small, funny stage direction: the master does not explain, he just points. 著 is zhe here, the ongoing-action marker — the finger stays raised while he speaks. This is the same character that opened the passage in a completely different job, which is why it is one of the hardest characters in the book to read aloud correctly.

Bajie laughed. "Filthy, that is what it is.

腌臢 is a wonderfully coarse word — grubby, revolting, the state of something you would not want on your hands. It survives in northern speech as āng zang, written 腌 with a different partner. Bajie's reaction to his brother's heroism is to think about the smell.