didn't need to do表示： 過(guò)去不必做某事, 事實(shí)上也沒(méi)做。.
needn't have done表示： 過(guò)去不必做某事, 但事實(shí)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(kāi)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(kāi)車去車站接瑪麗，所以她本不必步行回家了。 (Mary步行回家，沒(méi)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 hurried C. must not hurry D. needn't have hurried
答案D。needn't have done. 意為"本不必"，即已經(jīng)做了某事，而時(shí)實(shí)際上不必要。
Mustn't have done 用法不正確，對(duì)過(guò)去發(fā)生的事情進(jìn)行否定性推斷應(yīng)為couldn't have done, "不可能已經(jīng)"。 must not do 不可以(用于一般現(xiàn)在時(shí))。
