定语从句
定语从句修饰句中的某一名词或代词，用在先行词之后。引导定语从句的关系词有who, whom, whose, that, how, when, where, why和which。定语从句可以分为限制性定语从句和非限制性定语从句两种。
1． 限制性定语从句对所修饰的先行词起限制作用，在意义上与先行词密切不可分，一般不可用逗号主句分隔
[例句] A conductor is a substance which is able to carry electrons easily.
[译文] 导体是一种易于携带电子的物体。
[例句] Sources are often described in terms of the frequency that they occupy.
[译文] 信息源常用其信号所占用的频率来描述，
[例句] The reader who is impatient to get on to digital systems should realize that many digital systems also require analog technology to function.
[译文] 对于迫切想要了解数字系统的读者来说，应认识到数字系统中也需要用到模拟技术sort of翻译。
[例句] Such situations require the use of a carrier signal whose frequency is such that it will travel, or propagate, through the channel.
[译文] 这类情形需要使用载波信号，其频率使得它（指语音信号）可以通过信道传播。
[例句] Computing is an area where competition between companies is considerable and technology is moving very quickly.
[译文] 这是一个公司间竞争相当激烈而且技术进步非常快的领域。
注：当引导定语从句的先行词前有all, any, no, little, much, very, first 等词，或先行词前为形容词最高级、序数词所修饰时，或先行词为all, anything, nothing, everything 时，从句的引导词只能用that.
[例句] This is one of the very few books at any level that presents a formula for finding any voltage or current in the dc-excited RC circuits that are so important in timers.
[译文] 求解直流激励的RC或RL电路的电压或电流在定时器中是十分重要的，很少几本书给出了相应的公式，而本书即为其中之一。
[例句] Noise signals are any electrical signals that interfere with the error-free reception of the message-bearing signal.
[译文] 噪声信号就是任何对载信息信号的无误接收过程产生干扰的电信号。
2、非限制性定语从句仅对先行词作些描写或补充说明，有时含有原因、让步、时间、条件、结果等意思，从句要用逗号与主句分开，关系代词不能省略，通常用which (不用 that), who, when等引导。
[例句] Microwaves, which have a higher frequency than ordinary radio waves, are sued routinely in sending thousands of telephone calls and television programs across long distances.（表原因）
[译文] 微波比一般无线电波有更高的频率，因此通常用它传送大量长途电话和远距离电视节目。
[例句] Copper, which is used so widely for carrying electricity, offers very little resistance.
[译文] 铜的电阻很小，因此被广泛用于输电。（表结果）
[例句] Perhaps light is some sort of electric wave, whose nature we do not yet understand.
[译文] 也许，光是某种电波，其性质我们尚不清楚。
3．介词+which, whom, whose从句
[例句] Accurate measurement and computation are the necessary conditions upon which modern engineering and scientific research depend. （动词depend要求介词upon）
[译文] 精确的测量和准确的计算是现代工程和科学研究所以来的必要的条件。
[例句] The speed with which the waves travel is simply the rate at which the vibratory motion is passed on from particle to particle. （名词speed, rate分别要求介词 with, at）
[译文] 波的传播速度只不过是振动在粒子之间传递的速度。
[例句] A magnet can cause an electric current in a wire near which it is moved. （which指代wire 从句中的主语it指代magnet，根据全句含义确定介词near）
[译文] 磁铁能在使其运动所靠近的导线中产生电流。
4. 代词／名词+介词+which 从句，名词／代词是从句中的主语，of which　是主语的定语。
[例句] Direct current is an electric current the charges of which move in one direction only.
[译文] 直流电是其电荷只朝一个方向流动的电流。
[例句] The analog world is full of relative numbers, tradeoffs, and approximations, all of which depend heavily on the basic semiconductor properties.
[译文] 模拟领域充满了相对数、折衷和近似，所有这些都很大程度地取决于半导体的基本特性。
5. as 可做引导词引导定语从句, 多和such, the same 连用. As 引导的定语从句也可修饰整个句子, 既可放在先行词后,也可放在句子开头.
[例句] For a radio wave in free space, the velocity is the same as that of light, which is approximately 300~106 meters per second. .
[译文] 对于自由空间的无线电波，其传播速度与光速相同，大概在300~106米/每秒。

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