从句
　　从句分为定语从句，状语从句和名词从句三大类。定语从句又分为限定性和非限定性从句两种，由关系代词或关系副词引导，修饰主句的某个成分。状语从句分为时间、结果、让步原因、条件及行为方式状语从句。名词从句包括主语、宾语、表语和同位语从句及 there be 句型。
1. 定语从句
　　限定性定语从句中 that 可代表人和事，而 which 只代表事；二者在从句中作主语或宾语。 that 作宾语时常可省略， which 则不能，而且其后的 “ 不及物动词 + 介词 ” 中的介词不能省略。 which 作宾语时，先行词与 which 之间的介词不能省。例如：
　　The first doll that could say “mama”was invented in 1830.
    The second half of his voyage was by far the more dangerous part, during （不能省） which he sailed round the Cape Horn.
     代表 all, anything, something, nothing, much 等词时，用 that 而不用 which ， that 作宾语可省略。例如：
    You'd better do something he prefers to do to please him.
    That is the last time we met each other.
    I came across the woman you told me about yesterday.
    who 和 whom 引导的从句用来修饰人，分别作从句的主语和宾语。 where 是关系副词，用于表示地点的定语从句，而 when 用来表示时间。 whose 是关系代词，修饰名词作定语。例如：
    Henrey Ford is the person who is most responsible for developing the idea of mass production.
    The knee is the joint where the thigh bone meets the large bone of the lower leg.
    The next morning when she came down to breakfast, Mary was beaming with the pleasure ( 眉飞色舞 , 笑逐颜开 )of a new, great discovery.
    The visiting professor is a famous scientist whose son studies in my department.
  在下列情况，限定性定语从句的关系代词可以省略； 1 ）当关系代词在从句中作宾语或表语；或它们在从句中作介词的宾词，而该介词又在句末时； 2 ）当先行词是 that, all, only, everything, something, nothing 等代词时，或先行词前有一个最高级形容词修饰时； 3 ）当先行词前有 only, any, all, first, no, last 等修饰时，或先行词就是 time, moment, way(any-way) direction, distance 等名词时，关系代词即可省略。例如：
    The delegation we has been waiting for finally arrived
    I met the woman you told me about.
    She gave me all she could afford.
    With TV we can see things happen almost at the exact moment they are happening.
     非限定性定语从句，其作用为：对所修饰的成分作进一步的说明，它与主句用逗号隔开。此类从句省略后其余部分仍可成立。在非限定性定语从句中， which 可代表前面的整个句子；代表人时只能用 who, whom, 而不能用 that; as 也可用作关系代词。例如：
    The telephone, as we know, was the invention of Alexander Graham Bell.
    The Little White House in Warm Springs was the Georgia home of President Franklin D.Roosevelt, who died there on April 12,1945.
    Living in a damp house for a long time is harmful to one's health, which is known to everyone.
    2. 状语从句
     状语从句可分为：时间状语从句（主要由 when, whenever, after, before, as, since, once, as soon as, until, while ）  so……that such……that   though, although, no matter, even if however, whatever  as, because, since  for  is, whether, as long as, provided that where  as
    The small greenish flowers of the elm tree appear in the spring, long before the leaves grow.
    Qiong Yao's novels used to be so popular that her books appeared on best seller lists.
    Gas balloon is less safe than hot balloon because it may catch fire.
    Uniform acceleration( 同样的加速 )occurs if the rate of change remains the same over successive equal intervals of time.
    Thousands of Chinese college students went to the US Consulate in Shanghai, where they protested NATO's bombing at China's Embassy in Yugoslavia.
     状语从句的中 “ 主语 +be” 可以省略，前提是，从句主语和主句主语一致，且从句谓语 “be” 省略后的结构为 “ 连词 + 现在分词 / 过去分词 / 介词短词 / 形容词 / 名词短语 ” 。例如：
    When well fitted ， glasses can correct most sight defects in healthy eyes.
    If necessary, I would like to see you in your office.
    Although seriously wounded, he never complained

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