didnt need to do表示： 過去不必做某事, 事實上也沒做。.

　　neednt 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 didnt 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 neednt 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。

　　典型例題

　　There was plenty of time. She ___.

　　A. mustnt have hurried　 B. couldnt have hurried　　C. must not hurry　 D. neednt have hurried

　　答案D。neednt have done. 意為本不必，即已經做了某事，而時實際上不必要。

　　Mustnt have done 用法不正確，對過去發生的事情進行否定性推斷應為couldnt have done, 不可能已經。 must not do 不可以(用于一般現在時)。