need "不必做"和"本不必做"
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。
例如：
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步行回家了，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 hurried
C. must not hurry D. needn’t have hurried