从句从句按其在主句中的句法功能可分为三类：
即名词性从句、形容词性从句(即定语从句)和副词性从句(即状语从句)。
引导从句的词称作关联句。
一、名词性从句。
引导这些名词性从句的关联词包括：
从属连词that, if, whether;
连接代词who,whoever,whom,whomever,which,whichever,what,whatever,whose;
连接副词where, when, why, how。
其中， 从属连词只起连接作用， 在从句中不充当任何句法成分，而连接代词和连接副词既起连接作用， 在从句中又充当一定的成分。
that owen should h**e married his cousin is not at all surprising.
the fact is that he didn't go to the dinner party.
i don't know if he will attend the meeting.
h**e you heard the news that mary is going to marry tom?
你听说玛丽要和汤姆结婚的消息了吗？(that引导同位语从句)
提示]1. 在含有主语从句的复合句中， 为保持句子平衡， 常用it作形式主语，而将真正的主语从句置于句末。
it's well-known that water is indispensable to life.(形式主语)
2. 为保持句子平衡， that引导的宾语从句也常用it代替， 而将真正的宾语从句置于主句句末。这常常出现在主句有形容词或分词作宾语补足语的情况下。
he made it quite clear that he preferred to live here.
3. 从属连词whether和if都作“是否…”解， 但if不可引导主语从句和表语。
从句。whether可与or(not)连用， 而if不可以。
i don't know whether (if) she is at home.
whether she comes or not makes no difference.
4. that和what引导名词性从句的区别： that在从句中不充当成分，而what在从句中充当一定的成分， 如主语、表语、宾语等。that可省略， what则不可省。
he always means what he says.
she suggested (that) he do it at once.
5. 同位语从句大多由从属连词that引导， 常跟在下列名词后面， 如fact, idea, opinion, news, hope, belief等， that不可省。同位语从句一般用来解释说明这些名词的具体含义和内容。
we are familiar with the idea that all matter consists of atoms.
the news that we are invited to the conference is very encouraging.
二、定语从句。
引导定语从句的关联词包括：
1)关系代词：that, which, whose, who, whom, as;
2)关系副词：when, where, why。
关系代词和关系副词在定语从句中都充当一定的句法成分，关系代词在句中充当动词的宾语时， 一般可省去。
关系代词的选用比较复杂， 它受下列条件的制约：
1)先行词是指人还是指物；
2)关系代词在从句中的句法功能；
3)定语从句是限定性的还是非限定性的。
关系代词的选用情况见下表：
先行词在从句中的句法功能用于限定性或非限定性定语从句只用于限定性定语从句。
指人或指物指人指物。
主语who whichthat
宾语whom whichthat
定语whosewhose(of which)
i know that he is a man who (that) means what be says.
the gentleman whom (that) she met addressed her with courtesy(礼貌)。
the watch which (that) was lost has been found.
here is the meterial which (that) you need.
you're the only one whose advice he might listen to.
关系副词的选用相对来说较简单。如先行词为表示时间的名词， 如time, day等， 则用when; 如先行词为表示地点的名词， 如place, house, area等，则用where; 如先行词为reason, 则用why。
i'll never forget the mountain village where (in which) i spent my childhood.
i don't know the reason why (for which) he did that.
提示]1. 当先行词是all, something, nothing等不定代词时；或先行词前有first,last, only, few, much, some, any, no等修饰时；或先行词前有形容词最高级修时， 一般只用that而不用which来引导定语从句。
i've explained everything (that) i can to you.
this is the most beautiful compus (that) i've ever been to.
2. 定语从句分为限定性定语从句和非限定性定语从句， 限定性定语从句与主句关系紧密， 为句中不可缺少的部分， 如去掉， 主句意思不完整。非限定性定语从句与主句关系松散， 如去掉， 主句内容仍完整。
在书面语中， 非限定性定语从句一般被逗句隔开。非限定性定语从句一般不用that引导。引导非限定性定语从句的关系词不可省略。
his speech, which bored everyone, went on and on.
the general's daughter, whose name is ann, g**e me a sweet smile.
3. 先行词也可以是整个句子。这时， 一般用which或as来引导定语从句。
which在从句中可充当主语、宾语等， as在从句中一般只充当主语。which与as引导此类定语从句的区别在于： which只能置于句中或句末， 而as的位置比较灵活， 可置于句中、句末， 又可置于句首。
water consists of hydrogen and oxygen, which (as) is known to all.
he was awarded a gold medal, which the whole family considered a great honor.
as had been expected, he finished first in decathlon(十项全能) at that olympic games.
4. 关系代词在定语从句中有时也做介词的宾语。如果介词被置于关系代词前， 一般只用介词+which或介词+whom, 而不用介词+that来此导定语从句。
如果介词被置于从句句末， 则可用that代替which或whom,且that这时可省去。
this is the ring on which she spent 1,000 dollars.
one of my colleagues whom (that) you are familiar with will come today.
三、状语从句。
在复合句中， 起状语作用的从句称作状语从句。
根据语义， 状语从句分为：
时间状语从句、地点状语从句、条件状误从句、原因状语从句、
让步状语从句、比较状语从句、目的状语从句、结果状语从句、
方式状语从句。
状语从句可放在句首或句末。如状语从句位于主语前，一般用逗号将其与主句隔开。
一)时间状语从句。
when you cross a main road, you must be very careful.
until we learn the facts, we can't do anything about it.
提示]1. when, as, while引导时间状语从句时都可以表示主句的动作与从句的动作同时发生， 即同时性。它们的区别在于：
when和as引导的状语从句中的动作既可以是延续性的， 也可以是非延续性的， 即瞬时性的；
while引导的状语从句中的动作只能是延续的动作或状态。
当主句和从句中的动作均为延续动作时， 一般用while, 而不用when或as。当表示两个同时发展、变化的情况时， 一般用as, 作“随着…”解。
when she comes, i shall tell her to wait for you.
as she got older, she got wiser.
while peter was reading, his wife was cooking.
2. 有些副词和一些表示时间的名词词组也可用作从属连词，引导时间状语从句。如instantly, immediately, directly, the day, every time, the minute, the second, the moment等。
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