2015年职称英语考试常用语法详解:情态动词的完成式
情态动词的完成式：
can / may / should / ought to / must / need等动词称为情态动词。情态动词有三大特征：一是无人称和数的变化，二是后面跟的动词不带to，三是后面 + not构成否定式，比较：
He neednt go there so early.
You do not need to be an expert to make this simple design.
在讲解情态动词完成式前，先将一般式中应注意的几个问题提一下，以引起必要的重视。
1.情态动词一般式中应注意的几个问题：
注意以下用法的习惯与区别：
He must be a doctor.
否定：He cant be a doctor.
又如：They cant be working now.
May I go now?
回答：Yes, please. / Please dont.
have to 与 must
1) 当强调外界原因时，2)当与将来时结合时，用have to，
例如医学全在.线.提供, www.med126.com：
Since it is raining, we have to sty indoors.
You will have to hurry if you want to catch the 9:30 train.
3) 表示自然规律或必然结果，用must，
例如：
All men must die.
can 与be able to
can表示能力，但不一定做了或做成了;be able to 表示有能力并且做成了，相当于succeed in doing, 例如：
The patient is able to walk now.
The patient can walk now.
Shall I / we? 表示征求对方意见; + shall表示允诺或警告，例如：
Shall I fetch a doctor for you?
You shall be sorry for what you have done.
Will you.? 表示请求， 例如：
Don be late for the meeting next time, will you?
比较：I would remind you that smoking is forbidden here.
neednt 的肯定式为must，例如：
You neednt see him, but I must.
cannot but + 原形动词， 例如：
I cannot but tell you the truth.
2.情态动词的完成式：
情态动词的一般式用于表达现在或将来的情况，而情态动词的完成式则用来表达已经发生的事情，例如：你不应该来得那么晚。这是对已经发生的事情表示自己的看法：You shouldnt have come so late.
将常用、常考的几个情态动词完成式列表说明如下：
It must have rained last night; the ground is wet.(昨晚准是下雨了，地上湿漉漉的。)
The baby was cut badly; she shouldnt (oughtnt to) have let him play with the scissors. (孩子伤得很厉害;她不该让他玩剪刀。)
She may have known the news; she is crying. (她可能已经知道那个消息了;她在哭。)
You neednt have bought that dictionary; I have got a spare one. (你实际上没有必要买那本字典;我有一本多余的。)
由于在没有情景的情况下很难区分情态动词一般式和完成式，例如你不该去，究竟是You shouldnt go there. 还是 You shouldnt have gone there, 因此考试的时候往往会给出情景，这对判题非常重要，如上面表格例子中的the ground is wet，the baby was cut badly，she is crying, 都是题目所给的情景。