nowingly caused by an electrician.
2．表示虚拟语气。
1) needn’t have + 过去分词，表示做了不必做的事，相当于”didn’t need to do”,译为“其实没必要……”。如：
You needn’t have come over yourself.
As it turned out to be a small house party, we so formally.
[A] needn’t dress up [B]did not need have dressed up
[C] did not need dress up [D] needn’t have dressed up
(没有必要穿的那么正式，体现是说话者的建议，实际结果是否真的穿的很正式没有确定，答案为D)
2）should have +过去分词，表示应该做某事但实际上未做，译为“本应该……”should not + have过去分词表示本不应该做某事但实际上做了，译为“本不应该……”。如：
I regret having left the work unfinished; I should have planned everything ahead carefully.
我本来应该事先认真地把每件事情规划的很好，但实际上作者还是没有规划好，以至工作没有完成。
3) ought to have +过去分词，表示动作按理该发生了，但实际上未发生，译为“该……”，与should 的完成式含义类似。如：
Ｔhe porter ought to have called the fire-brigade as soon as he saw the fire in the stock, which went up in smoke .
4) could have +过去分词，表示过去本来可以做但却未做，译为“完全可以……”。这点与ought/should/ have +过去分词用法相似。如：
What you said is right, but you could have phrased it more tactfully.
5) may/ might have +过去分词，表示过去可以做但实际未做，译为“（那样）也许会……”。如：
It might have been better to include more punchy statistics and photos of equipment in the introduction to further assist first-time office automation managers.
二．几个情态动词常考的句型：
1)．may/might (just) as well “不妨，最好”，与had better相近；
Since the flight was cancelled, you might as well go by train.
既然航班已经取消了，你不妨乘火车吧。相当于you had better go by train。
2) ．cannot / can’t…too …“越……越好，怎么也不过分”。注意这个句型的变体cannot…over….如：
You cannot be too careful when you drive a car.驾车时候，越小心越好。
The final chapter covers organizational change
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