这个结构用来表示虽然不必要但却已完成的动作:
I neednt have written to him because he phoned me shortly afterwards.
我本来不需要给他写信,因为不久之后他就给我打了电话。(但我的确写了,因而浪费了时间。)
You neednt have brought your umbrella for we are going by car.
你本来不用带雨伞,因为我们将要坐车去。(但是你却多余地带了伞。)
He neednt have left home at 6:00;the train doesnt start till 7:30.
他本不必6点就离开家;火车7点30分才出发。(因此他要等一个小时。)