公主諫阻 The Princess's Warning
公主道：「我父王不是馬掙力戰的江山，他本是祖宗遺留的社稷。自幼兒是太子登基，城門也不曾遠出，沒有見你這等兇漢。你這嘴臉相貌，生得這等醜陋，若見了他，恐怕嚇了他，反為不美。卻不如不去認的還好。」老妖道：「既如此說，我變個俊的兒去便罷。」公主道：「你試變來我看看。」公主道：「我父王不是马挣力战的江山，他本是祖宗遗留的社稷。自幼儿是太子登基，城门也不曾远出，没有见你这等凶汉。你这嘴脸相貌，生得这等丑陋，若见了他，恐怕吓了他，反为不美。却不如不去认的还好。」老妖道：「既如此说，我变个俊的儿去便罢。」公主道：「你试变来我看看。」
逐句解 Line by line
The princess said: "My royal father's realm was not won by warhorse and force of arms; his is a domain and altar handed down from the ancestors.
Her argument is a political one, and it is exact. 馬掙力戰 — struggling on horseback, fighting by main strength — is how a throne is taken by conquest. 社稷, the altars of soil and grain, is the standard stand-in for the dynastic state itself. Her father inherited; he did not fight. The distinction matters, because it means he has never seen anything like her husband.
From his childhood he came to the throne as crown prince; never once has he ventured far beyond the city gates, and he has looked upon no such fearsome brute as you.
太子 takes 子 as zǐ, the noun "crown prince." In the colloquial 自幼兒, "from his young days," the 兒 is er. A sheltered king, on the throne since childhood, who has never been far outside his own city gate.
With a face and features like yours, born so hideous, should he set eyes on you I fear you would frighten him — and that would turn out ill.
相貌, features, physiognomy, takes 相 as xiàng; 生得, "born so, formed so," takes the structural 得 as de; and 反為不美, "would turn out badly after all," takes 為 as wéi, "to be, to turn out." She does not say he would be caught out as a demon. She says he is too ugly to be let in.
Better, then, not to go and claim kin at all."
不如, "better not to"; 還好 takes 還 as hái, "all the same better."
The old fiend said: "If that is how it stands, I shall simply change into a handsome fellow and go."
In 俊的兒, "a handsome one," the 兒 that turns the phrase into a noun is er. He treats changing his entire body the way anyone else might treat changing a shirt.
The princess said: "Then try the change, and let me see."
"Try it, and let me look." Her prompt is the cue, and it is a standard one: vernacular fiction habitually frames a transformation with just such an invitation, so that the long descriptive set-piece which follows arrives as an answer to somebody's question rather than as an author clearing his throat.
The princess's warning is politically precise: her father's throne is an inherited 社稷 and not a conquest, and a king who has never left his own city has no experience of monsters. The danger she identifies is exposure by sheer ugliness — a physical tell, not a moral one.
His airy remedy — 變個俊的兒, "change into a handsome one" — treats the transformation as nothing at all, and her "try it and let me see" is the conventional cue for the portrait in verse that follows. Plot and ornament are stitched together neatly here: her request is also the reader's.