纸上谈兵 Paper tiger
Explanation
比喻空谈理论，不能解决实际问题。也比喻空谈不能成为现实。
It means talking about theories without solving practical problems. It also means that empty talk cannot become reality.
Origin Story
战国时期，赵国名将赵奢的儿子赵括，从小熟读兵书，谈起军事战略头头是道，就连他父亲也辩不过他。赵括自认为军事才能天下无敌，然而，他所有的军事知识都来自书本，缺乏实战经验。后来，赵王不听劝阻，任命赵括代替老将廉颇统帅军队。在长平之战中，赵括墨守成规，不懂得随机应变，最终导致赵军惨败，四十万大军全军覆没，赵国元气大伤。这个故事告诉我们，纸上谈兵，只会导致失败，只有理论联系实际，才能取得成功。
During the Warring States period, Zhao Kuo, the son of the famous Zhao general Zhao She, was a voracious reader of military texts. He could discourse on military strategies with great authority, even his father could not out-argue him. Zhao Kuo believed his military prowess was unparalleled; however, his knowledge was solely derived from books, lacking practical experience. Later, disregarding counsel, the Zhao king appointed Zhao Kuo to replace the veteran general Lian Po as the commander-in-chief. In the Battle of Changping, Zhao Kuo rigidly adhered to doctrines, failing to adapt to the changing circumstances. This ultimately led to the catastrophic defeat of the Zhao army, resulting in the annihilation of 400,000 soldiers and the crippling of the Zhao state. This story illustrates that theoretical knowledge alone leads to failure; only by combining theory with practice can success be achieved.
Usage
作谓语、宾语、定语；指空谈；比喻只说不做，脱离实际。
As a predicate, object, attributive; refers to empty talk; metaphor for only talking without doing, detached from reality.
Examples
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他总是纸上谈兵，缺乏实际操作经验。
tā zǒngshì zhǐ shàng tán bīng, quēfá shíjì cāozuò jīngyàn
He always talks big, lacking practical experience.
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不要纸上谈兵，要拿出实际行动来。
bùyào zhǐ shàng tán bīng, yào ná chū shíjì xíngdòng lái
Don't just talk the talk, walk the walk.
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空想家们往往纸上谈兵，却做不出什么实际成果。
kōngxiǎng jiāmen wǎngwǎng zhǐ shàng tán bīng, què zuò bù chū shénme shíjì chéngguǒ
Dreamers often just talk the talk but achieve nothing tangible