A needn't have done意指没必要干某件事但多余地干了，即浪费了时间：

You needn't have watered the flowers，for it is going to rain．

你本不必浇那些花，因为就要下雨了。（你浪费了时间。）

You needn't have written such a long essay．The teacher only asked for

300 words，and you have written 600．

你本不必写这么长的文章。老师只要求写300个字而你却写了600个字。

He needn't have bought such a large house．His wife would have been quite happy in a cottage．

他没必要买这么大一所房子。他的妻子住在一个小房子里也会感到很满意的。（浪费了钱）

You needn't have carried all these parcels¹ yourself．The shop would have delivered them if you had asked them．

你本来没有必要自己拿着这些大包小裹的。要是当时你跟商店说一声，他们就会把东西送来的。

B didn't have／need to do意指不一定非干某事，而通常也并没有这样做：

I didn't have to translate it for him for he understands Dutch²．

我用不着替他翻译，因为他懂荷兰语。

I didn't have to cut the grass myself．My brother did it．

我用不着自己修剪草坪。我的兄弟修剪过了。（无义务，而且无行动）有些人的确把didn't have to／didn't need to用于表示已完成的动作，这时 have或 need都要加以重读：You didn't 'have to give him my name通常意味着没有必要告诉他我的姓名，但是你却告诉了他。但当一个不必要的动作实际已完成了时，还是用needn't have+过去分词为好：

You needn't have given him my name．

你本来没必要把我的姓名告诉他的。