刮目相看 to view with new eyes
Explanation
形容对人的才干、能力有了新的认识，改变了过去的看法。
Describes a new understanding of a person's abilities and talents, changing the past view.
Origin Story
三国时期，吴国大将鲁肃素来轻视吕蒙的军事才能，认为他只会舞刀弄枪，不通文墨。后来，孙权劝吕蒙学习，吕蒙果然发奋读书，学识大增。鲁肃再次见到吕蒙时，发现他谈吐不凡，对军事战略、兵法韬略都有独到的见解。鲁肃十分惊讶，感叹道：“士别三日，即更刮目相待！”意思是说，即使是相识很久的朋友，如果彼此之间有了很大的变化，也应该重新看待对方。这个故事就演变成了成语“刮目相看”，用来形容对人有了新的认识，改变了过去的看法。
During the Three Kingdoms period in China, Wu general Lu Su had always looked down upon Lü Meng's military capabilities. He believed Lü Meng was only skilled in martial arts, lacking in literary knowledge. Later, Sun Quan encouraged Lü Meng to study, and he diligently applied himself, greatly expanding his knowledge. When Lu Su met Lü Meng again, he was astonished to find that Lü Meng was articulate and had insightful views on military strategy and tactics. Lu Su exclaimed in surprise, "士别三日，即更刮目相待！" (Shi bie san ri, ji geng gua mu xiang dai!), meaning that even long-acquainted friends should be reassessed if they've undergone significant changes. This story evolved into the idiom "刮目相看" (gua mu xiang kan), used to describe the changing of one's view of someone due to their progress.
Usage
用于赞扬取得很大进步的人，或表示对人有了新的认识。
Used to praise people who have made great progress, or to express a new understanding of someone.
Examples
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他这次的表现让我们刮目相看。
tā zhè cì de biǎo xiàn ràng wǒ men guā mù xiāng kàn
His performance this time has really impressed us.
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经过几年的努力，他已经让我们刮目相看了。
jīng guò jǐ nián de nǔ lì, tā yǐ jīng ràng wǒ men guā mù xiāng kàn le
After years of hard work, he has really impressed us.
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他的进步如此之大，真让我们刮目相看！
tā de jìn bù rú cǐ zhī dà, zhēn ràng wǒ men guā mù xiāng kàn
His progress is so great that it's really impressive!