这个结构用来表示虽然非必要但却已完成的动作:
I needn't have written to him because he phoned me shortly1 afterwards.
我本来无需给他写信,由于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 needn't have brought your umbrella for we are going by car.
你本来不需要带雨伞,由于大家将要坐车去。(但你却多空间带了伞。)
He needn't have left home at 6:00;the train doesn't 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。)