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Complete the student's diary entry about a lucky break. Select ALL the options that logically and grammatically fit the blank to mean she knew the exam was canceled beforehand, so she did not study.
I checked my email on Friday and saw the exam was canceled! Therefore, I ________.
The correct answers are didn't need to study and didn't have to study.
When an action was unnecessary and therefore not performed, we use didn't need to or didn't have to.
If she had said "I needn't have studied," it would mean she actually spent hours studying and only found out later that the exam was canceled! (Also, "needn't to" is always grammatically incorrect because "needn't" is followed by a bare infinitive).