判若两人 as different as two people
Explanation
形容一个人前后表现、态度变化很大，就像两个人一样。
Describes a person whose behavior and attitude have changed dramatically, as if they were two different people.
Origin Story
从前，在一个小山村里，住着一位名叫小明的年轻人。他平时沉默寡言，性格内向，喜欢独自一人看书学习。村里人都说他是一个老实巴交的人，从不惹是生非。然而，有一天，小明突然变了个人似的，变得异常活泼开朗，整天和朋友们嬉戏打闹，甚至还学会了喝酒。这种变化让村里人都感到惊讶不已，他们不明白小明为什么会发生如此巨大的转变。原来，小明最近迷上了一部武侠小说，小说中主人公的侠肝义胆深深地吸引了他。他开始模仿小说中的人物，试图让自己变得勇敢、自信，也试图摆脱过去那种沉默寡言的性格。但是，他这种变化并没有持续多久，因为这只是他的一时兴起，很快他就又恢复了原来的模样。虽然小明的变化仅仅是一时的，但是却足以说明一个人在不同环境和心境下的表现差异之大，也足以说明“判若两人”这个成语的含义。
Once upon a time, in a small mountain village, there lived a young man named Xiaoming. He was usually quiet and introverted, preferring to spend his time reading and studying alone. The villagers described him as an honest and simple man who never sought trouble. However, one day, Xiaoming suddenly changed. He became unusually lively and outgoing, playing all day with his friends and even learning to drink alcohol. This change greatly surprised the villagers. They couldn't understand why Xiaoming had changed so much. It turned out that Xiaoming had recently become fascinated with a wuxia novel. The novel's protagonist, with his courage and righteousness, deeply attracted him. He began to imitate the character in the novel, trying to make himself brave and confident, and trying to break free from his previous quiet and introverted personality. But this change didn't last long, as it was only a passing fancy. Soon he returned to his old self. Although Xiaoming's change was only temporary, it is sufficient to illustrate the great difference in a person's behavior in different environments and moods, and is sufficient to illustrate the meaning of the idiom "判若两人".
Usage
用于形容一个人前后表现截然不同，如同两个人一般。常用于贬义，表示不满或嘲讽。
Used to describe a person whose behavior before and after is completely different, like two different people. Often used in a derogatory sense, to express dissatisfaction or ridicule.
Examples
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他今天的表现判若两人，上午还萎靡不振，下午却精神抖擞地完成了任务。
tā jīntiān de biǎoxiàn pàn ruò liǎng rén, shangwǔ hái wěimí bù zhèn, xiàwǔ què jīngshén dǒusǒu de wánchéng le rènwu。
His performance today was like two different people; in the morning, he was listless, but in the afternoon, he completed the task with great vigor.
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这场比赛中，他判若两人，上半场表现糟糕，下半场却技惊四座。
zhè chǎng bǐsài zhōng, tā pàn ruò liǎng rén, shàng bàn chǎng biǎoxiàn zāogāo, xià bàn chǎng què jì jīngsìzuò。
In this game, he was like a different person; he performed poorly in the first half, but in the second half, he amazed everyone with his skills.