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全国职称英语考试语法知识分类讲解及对应习题(六)

分类: 职称英语 

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 needn't go there so early. (他不需要去那么早。- 情态动词)

You do not need to be an expert to make this simple design. (这么简单的设计不一定非要专家来做。- 动作动词)

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. (他准是个医生。)

否定:He can't be a doctor. (他不会是个医生。- 注意:一般不用must' be )

又如:They can't be working now.(现在他们不可能在工作。)

May I go now?

回答:Yes, please. / Please don't. (或:No, you mustn't)

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:

Since it is raining, we have to sty indoors.

You will have to hurry if you want to catch the 9:30 train. (你要是赶9:30的火车,得赶紧。)

3) 表示自然规律或必然结果,用must,

例如:

All men must die. ( 人固有一死。)

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. (说明病人原来是丧失了走的能力的,现在自己能走了)

The patient can walk now. (可能的情况是,病人是能走的,但医生不让他走,如急性心梗后医生不让病人走动。)

Shall I / we(第一人称)…? 表示征求对方意见;(第二、三人称) + shall…表示"允诺"或"警告,例如:

Shall I fetch a doctor for you? (要我为你请个医生来吗?)

You shall be sorry for what you have done. (你会为你所做的事后悔的。)

Will you….? 表示请求, 例如:

Don't' be late for the meeting next time, will you? (下次开会别迟到了,好吗?)

比较:I would remind you that smoking is forbidden here. (表示"婉转的建议或请求":我想提醒你,这里不能抽烟。)

needn't 的肯定式为must,例如:

You needn't see him, but I must. (你不必去看他,但我得去。)

cannot but + 原形动词 (意思相当于have to), 例如:

I cannot but tell you the truth. (我不得不给你说实话。)

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(你来晚了)表示自己的看法:You shouldn't 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:

must (not) have done表示对已发生的事情的"判断",中文常表达为:"准是…"It must have rained last night; the ground is wet.(昨晚准是下雨了,地上湿漉漉的。)

should (not) have done / ought (not) to have done表示"事与愿违",中文常表达为:"(不)该…","本(不)该…"The baby was cut badly; she shouldn't (oughtn't to) have let him play with the scissors. (孩子伤得很厉害;她不该让他玩剪刀。)

may / can (not) have done表示"本(不)可能" She may have known the news; she is crying. (她可能已经知道那个消息了;她在哭。)

need (not) have done表示"本(不)需要"You needn't 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 shouldn't go there. 还是 You shouldn't 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。

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