The Great Sage, guarding the Tang Monk, led the horse and kept watch over the luggage.

大聖, 'the Great Sage,' is short for 齊天大聖, 'Great Sage Equal to Heaven' — the title Wukong awarded himself. And here is the Great Sage Equal to Heaven holding a bridle and minding the bags. 牽著馬, 'leading the horse,' with 著 as zhe, the marker for an action that goes on and on. Going on and on is precisely the trouble.

Seeing Bajie locked in combat with the monster, he ground his teeth in vexation, rubbed his palms and clenched his fists, and could not hold back from joining the fray;

恨得咬牙切齒, 'so galled he ground his teeth' (得 de), and 擦掌磨拳, rubbing the palms and grinding the fists. Both are still in daily use in Chinese, and the second is exactly what English means by itching for a fight. Here they are applied to an actual ape, on sentry duty, watching somebody else have all the fun.

He drew out his rod and said: 'Master, sit tight and have no fear. Wait while old Sun goes and has a bit of sport with him.'

你坐著, 'you sit tight' (著 again zhe). Then 耍耍兒: 'to go and have a bit of a play with him.' A demon is trying to eat his master and the monkey calls it going out to muck about. The next several pages exist to puncture that attitude.

The master could by no means hold him back. He gave a whistle and bounded to the front.

苦留不住: the master pleaded and could not hold him. 打個唿哨, 'gave a whistle,' is Wukong's habitual noise on launching himself at something — the sound effect that means he has stopped listening.

Now the monster and Bajie were just then at the height of their contest, so evenly matched that neither could break free.

戰到好處, 'fighting at just the right pitch,' and 難解難分 (難 nán), 'hard to unlock, hard to part.' That second phrase is still the ordinary Chinese for two things too tangled to pull apart, whether a brawl or a love affair. The fight is genuinely even — which is what makes the next line so infuriating.

Then the Pilgrim swung up his iron rod and brought it down full on the monster's head;

掄起鐵棒, 'whirled up the iron cudgel' — the 如意金箍棒, the gold-banded wishing staff. 著頭一下 is 'one on the head,' and here 著 is zháo, the reading for a blow that actually connects. He is not sparring. He means to kill it.

The monster spun about in haste, dodged clear in a fluster, and dived straight down into the River of Flowing Sands.

鑽入流沙河裡: bored straight down into the river. That one move is the whole problem of the chapter. The demon's element is water and the monkey's emphatically is not, so he cannot follow. Everything that happens for the next several scenes is an attempt to get round this single sentence.

This so angered Bajie that he hopped about, crying: 'Brother! Who told you to come?

氣得個, 'so furious that' (得 de) — and then the character worth watching. 誰著你來的, 'who sent you?', reads 著 as zhuó, the vernacular sense of ordering somebody to do a thing. That makes three of this one character's four readings inside a single quarrel: zhe for the horse being led and the master sitting still, zháo for the cudgel landing, zhuó for being sent. One character, three jobs, one scene.

'The monster was flagging by degrees, hard put to fend off my rake; in three or five more rounds I'd have had him fast.'

Bajie's grievance is specific, and the narration half believes him. The monster was tiring — 難架我鈀, 'scarcely able to block my rake' (難 nán) — and 再不上三五合, 'in no more than three or five further rounds,' he would have had him.

'Now that he has seen how fierce you are, he has broken off and fled — however are we to manage?'

兇險, 'fierce and dangerous,' is what the demon saw in the monkey; 怎生是好, 'however is this to be put right?', is where it leaves them. The complaint has become the practical question: the quarry is now somewhere neither of them can go.

The Pilgrim said with a laugh: 'Brother, I'll tell you the plain truth:

實不瞞你說, 'I'll not hide it from you' — the monkey's standard opening for an admission he is going to enjoy making.

'ever since I subdued the Yellow Wind fiend and we came down the mountain, I've not had my cudgel out to play for a good month;

降了黃風怪, 'subdued the Yellow Wind fiend' (降 is xiáng, to subdue, not jiàng, to descend), points back to the ordeal of the last chapter. Since then: 這個把月, 'a month or so,' and 不曾耍棍, 'no play with the staff.' He is bored. That is the entire explanation.

'seeing you and him going at it so prettily, I couldn't keep my feet from itching, and so I bounded over for a bit of sport.'

戰的甜美 — the fight looked 'sweet' to him, which from this quarter is a compliment. 腳癢, 'itchy feet,' is the excuse, and 跳將來, 'came bounding over' (將 jiāng, the small particle that shoves a verb in a direction). He is wholly unrepentant, and rather charming about it.

'How was I to know the monster couldn't take a joke, and off he went.'

那知, 'who was to know' — and here 那 is nǎ, the question word, sitting two characters away from 那怪, 'that monster,' where it is nà, the pointing word. Then the punchline: 不識耍, 'he couldn't take a joke.' A duel to the death, ruined by the other fellow's poor sense of humour.