“板上钉钉”，汉语成语，本义指在石板上钉上铁钉（nail the slate with nails），比喻事情已经决定，不能改变或事情已经成了事实。与英文惯用语“cast in stone”意思相近，表示“Completely set, unchangeable; not subject to any amendment or alteration”。

例句：

他只是提出这个观点供讨论，事情还没板上钉钉。

He is merely throwing the idea forward for discussion. It is not cast in stone.

虽然我很想参加，但是我得去出差。而且我知道，日期已经不能再改，是板上钉钉的事了。

While I really want to attend the party, I'll be on a business trip. And I understand the date of the ceremony can't be changed. It's cast in stone.