As the students only made a few mistakes,they __ (need not punish) so severely.我填了don't need to punish 错了，正确答案为：need't have been punished请教一下，为什么要这么做，这两者的区别，谢谢!
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注意2点：1 students 是被punish, 所以要用被动语态 be punished2 need't have done 是对过去的虚拟语气，意思是“本不需要做某事”。因为学生犯的错误很少，他们本不需要被那么严厉地惩罚的。