青石板上钉钉 ——— 不动
前一个“钉” (dìng) 是动词，后一个“钉” (dīng) 是名词。在坚硬的石板上钉钉子，钉子是钉不进去的，也就是“钉 (dìng) 不动”。这个歇后语用来比喻事情发展已成定局，基本上不会再发生变动。
Driving a nail into a stone slab — impossible to penetrate
The first 钉 is a verb, while the second one is a noun. A nail cannot penetrate a stone slab, no matter how hard one drives it. In Chinese it is called 钉不动, which means that it is impossible to drive a nail into something. The extended meaning of this idiom is that things are already settled and cannot be changed.