板上钉钉 a done deal
Explanation
比喻事情已经决定，不能改变。
It means that something is already decided and will not change.
Origin Story
从前，有个木匠，他接了一个制作精巧木盒的订单。为了确保万无一失，他特意选用上好的木材，用铁钉将木板牢牢钉在一起。每钉一颗钉子，他就仔细检查，确保钉子稳固，不会松动。盒子的制作过程十分细致，木匠一丝不苟，每一个步骤都力求完美。终于，精美的木盒完成了，木匠看着结实牢固的木盒，满意地笑了。这个木盒就像他人生中许多事情一样，只要用心去做，就能做到板上钉钉。
Once upon a time, there was a carpenter who received an order to make a very elaborate wooden box. To ensure success, he chose high-quality wood and used iron nails to firmly secure the boards together. With each nail, he would carefully inspect to make sure it was secure and wouldn't come loose. The box's construction was incredibly detailed; the carpenter was meticulous, striving for perfection in every step. Finally, the beautiful wooden box was finished, and the carpenter smiled contentedly, looking at the sturdy box. This box was like many things in his life: if you put your heart into it, you can make it a done deal.
Usage
用于形容事情已成定局，无法改变。常用于正式场合。
Used to describe something that is a done deal and cannot be changed. Often used in formal situations.
Examples
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这件事已经板上钉钉了，不会再改变了。
zhè jiàn shì qíng yǐ jīng bǎn shàng dìng dīng le,bù huì zài gǎi biàn le.
It's a done deal; it won't change.
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他的升职已经板上钉钉，就等通知了。
tā de shēng zhí yǐ jīng bǎn shàng dìng dīng,jiù děng tōng zhī le
His promotion is a done deal; he's just waiting for the notification.