畫虎不成 Painting a Tiger and Getting a Dog
王子道：「願使棍的就學棍，慣使鈀的就學鈀，愛用杖的就學杖。」行者笑道：「教便也容易，只是你等無力量，使不得我們的兵器，恐學之不精，如畫虎不成反類狗也。王子道：「愿使棍的就学棍，惯使钯的就学钯，爱用杖的就学杖。」行者笑道：「教便也容易，只是你等无力量，使不得我们的兵器，恐学之不精，如画虎不成反类狗也。
逐句解 Line by line
The princes said, "Let the one who wishes to use the staff learn the staff, the one used to the rake learn the rake, and the one who likes the cudgel learn the cudgel."
Three clauses of identical shape with one weapon swapped in each, and it is the sentence that pairs boy to master. The eldest carried a staff out of the palace, the second a rake, the third a black cudgel, and each now asks for the matching art. 願, 慣, 愛 — wishes to, is used to, likes — vary just enough to keep it from being mechanical.
Pilgrim laughed and said, "Teaching is easy enough. It is only that you have not the strength — you cannot handle our weapons, and I am afraid the learning would not take, so that you would set out to paint a tiger and end with a dog.
教 is jiāo here, to teach a thing to somebody, and this is the same character that was the ordinary causative eleven other times in the chapter. 得 is de in 使不得, the can-or-cannot hinge: cannot manage them. And 畫虎不成反類狗 is a real proverb of some antiquity — set out to paint a tiger and produce something more like a dog. It is still current, and it means overreaching and producing an embarrassment.
The princes' request is the chapter's design made explicit. Each of them names the weapon he already favours, and each of those is the weapon of one of the three disciples.
Pilgrim's objection is technical rather than moral. He does not say they are unworthy; he says they are not strong enough, and that a half-learnt art is worse than none.
The tiger-and-dog proverb is the sort of thing this novel loves — a piece of ordinary wisdom, still in daily use, dropped into a conversation between a monkey and three princes about supernatural strength.