didnt need to do表示： 过去不必做某事, 事实上也没做。
neednt have done表示： 过去不必做某事, 但事实上做了。
John went to the station with the car to meet Mary, so she didnt need to walk back home. 约翰开车去车站接玛丽，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. 约翰开车去车站接玛丽，所以她本不必步行回家了。 (Mary步行回家，没有遇上John的车。)
典型例题
There was plenty of time. She ___.
A. mustnt have hurried B. couldnt have hurriedC. must not hurry D. neednt have hurried
答案D。neednt have done. 意为本不必，即已经做了某事，而时实际上不必要。
Mustnt have done 用法不正确，对过去发生的事情进行否定性推断应为couldnt have done, 不可能已经。 must not do 不可以（用于一般现在时）。