didn't need to do表示： 過去不必做某事, 事實上也沒做。.
needn't have done表示： 過去不必做某事, 但事實上做了。
john went to the station with the car to meet mary, so she didn't need to walk back home. 約翰開車去車站接瑪麗，所以她不必步行回家了。
john went to the station with the car to meet mary, so she needn't have walked back home. 約翰開車去車站接瑪麗，所以她本不必步行回家了。 (mary步行回家，沒有遇上john的車。)
典型例題
there was plenty of time. she ___.
a. mustn't have hurried b. couldn't have hurriedc. must not hurry d. needn't have hurried
答案d。needn't have done. 意為"本不必"，即已經做了某事，而時實際上不必要。
mustn't have done 用法不正確，對過去發生的事情進行否定性推斷應為couldn't have done, "不可能已經"。 must not do 不可以（用於一般現在時）。