“What's done cannot be undone”
Embedded and contextualised example: At the end of Act 5 Scene 1, Lady Macbeth returns to bed, but before she does so she concludes her rambling final scene by claiming that “what’s done cannot be undone.”
It’s short and easy to learn
It is perhaps the most important structural echo in a scene full of structural echoes: in Act 3 Scene 2 she said, “What’s done is done” - though very similar in terms of the words used, the difference in meaning between these two lines is dramatic: before she wanted to put the past behind her; now she wishes to change the past but knows she cannot
Relevant characters and themes: Lady Macbeth, guilt and remorse, power and corruption, greed and ambition, good and evil