常用结构有：

be in need of ... 需要…

feel the necessity of ... 感到有…的必要

it is necessary / essential + (for ...)to V 必须…

make it necessary / unnecessary + to V 使…成为必要／不必要

make necessary / unnecessary + NP 需要／不需要

must / should + V 必须…

need / require / necessitate + NP 需要／要求／使成为必需(必要)

obviate / dispense with / do away with the need for ...

消除了对…的需要，无需…

there is no need / necessity of / for / to V 没有…的必要，无需…

Some cities are in urgent need of new water supplies .

有些城市急需新的供水水源。

To understand how semiconductors work it is necessary to review briefly the structure of matter．

要了解半导体器件如何工作，必须先粗略地了解一下物质的结构。

It is essential for the process to be carried out quickly .

这一过程必须很快地进行。

This type of engine needs / requires very little maintenance .

这种发动机需要的维修保养工作很少。

The production of this new model will necessitate complete retooling of factory .

生产这种新的型号(汽车)将需要对工厂进行重新装备。

Half-life is the time required for a radioactive substance to be reduced to 50 percent of its original radioactivity .

半衰期是放射性物质的放射性被降到原来的一半所需的时间。

A large area of heating surface is necessary to produce the required weight of steam.

要生产所需重量的蒸汽，需要有大面积的受热面。

The use of plastic pipes makes unnecessary / obviates the need for protection against corrosion．

使用塑料管子就不必考虑防腐蚀的问题了。

A force of attraction then acts between the two poles , making it necessary to exert a force to separate them .

这时两极之间有一力作用，因而需要施加一个力才能把两个极分开。

The bearings must be lubricated．

轴承需要加以润滑。

To ensure freedom from distortion , it is necessary for the metal bar to be cooled slowly .

为了保证不发生变形，金属棒必须慢慢冷却。

There is no need for any readjustment after operation .

开动后无需再作调整。

责任编辑：admin