Final answer:
John Proctor forgets the commandment 'Thou shalt not commit adultery' in Act 2 of 'The Crucible'. This omission is dramatically ironic considering his past affair with Abigail Williams.
Step-by-step explanation:
In Act 2 of Arthur Miller's play The Crucible, John Proctor forgets one of the Ten Commandments when Reverend Hale quizzes him. The specific commandment Proctor fails to remember is "Thou shalt not commit adultery." This lapse is particularly ironic given Proctor's earlier sinful relationship with Abigail Williams, which is a key element of the play's conflict.
The fact that this is the commandment he forgets is rich with dramatic irony, as it highlights Proctor's internal struggle and the themes of guilt and hypocrisy that are central to the narrative of The Crucible. It also foreshadows the severe consequences this sin will have on the lives of Proctor and other characters as the play progresses.