I can only help you with the meaning...
It means you can't change your past actions. It's usually used when someone makes a mistake and then has regrets about their actions.
1. If you hurt someone's feelings, you may wish that you could take back your words, but you can't.
2. If you break a drinking glass, you can't undo the action.
3. If you accidently kill someone, you can't bring them back to life.
Often, this phrase is used to tell the person who made the mistake to stop thinking about what they wish they had done and to begin concentrating on what they're going to do next to fix the problem.
Good luck on the grammar,
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