2017-12-29 发布 924 次浏览


adj. 必要的;强制的;必然的;不可避免的

n. 必需品;


1. 在系表结构后接不定式时,不能按汉语意思用“人”作主语,而用形式主语it。如:


误:He’s necessary to go with us.

正:It’s necessary for him to go with us.

正:It’s necessary that he (should) go with us.

在以上后接 that 从句的句型中,从句谓语通常要用虚拟语气。又如:

It’s necessary that he (should) buy a computer. 他有必要买台电脑。

It’s necessary that we (should) ask for her advice. 我们有必要去征求一下她的意见。

2. 表示“对……有必要”,其后可接介词 to 或 for。如:

Food is necessary for [to] life. 食物对生命是必要的。

Sleep is necessary to [for] one’s health. 睡眠对健康是必不可少的。

当后接不定式的复合结构时,引导不定式逻辑主语的介词通常只用for而不用 to。如:

It’s necessary for us learn a foreign language. 我们有必要学习一门外语。

3. 可与 if, when, where, as, whenever, wherever, although 等连词构成省略句(可以看成是其中省略了it is)。如:

If necessary, ring me at home. 如果必要,可往我家里打电话。

Tell him all about it when necessary. 在必须的时候把一切都告诉他。

Where necessary, improvements will be made. 哪儿需要,就在哪儿改进。

They believed in the application of force wherever necessary. 凡属必要的地方他们都主张使用武力。

另外,注意习语as necessary(按需要,根据需要),than necessary(比需要的更)。如:

We’ll be adding more workers as necessary. 我们会按需要增加更多的工人。

I drove ten miles farther than necessary. 我开车开过了10英里。


1. For her, books were as necessary to life as bread.


2. Now there is a dawning realisation that drastic action is necessary.


3. Fry remaining peppers, adding a little more dressing if necessary.


4. It's only necessary to use just a very small blob.


5. We will use air power to protect UN peacekeepers if necessary.


6. Carbon dioxide is a necessary result of the oxidation of carbon compounds.