主从复合句是指含有两个或更多主谓结构的句子，其中有一个（或更多）主谓结构充当整个句子的某一（些）成分。充当定语的句子叫定语从句，充当状语的句子叫状语从句，充当主语、宾语、表语或同位语的句子叫名词性从句。主句与从句之间用关联词连接起来。关联词除了在主句与从句之间起连接作用外，绝大多数不但本身有意义，而且还要在从句中充当句子成分。但是，引导名词性从句的whether或if有“是否”之意，但不在从句中充当句子成分，‘引导名词性从句的that没有任何意义，也不在从句中充当句子成分。

主从复合句由一个主句和一个或一个以上的从句构成。主句为句子的主体,从句只用作句子的一个次要成分,不能独立成为一个句子。从句通常由关联词引导,并由关联词将从句和主句联系在一起。如：While the grandparents love the children, they are strict with them.(爷爷奶奶们很爱孩子,同时对他们也严格要求。) / It seemed as if the meeting would never end.(看起来会议没完没了。) / Hurry up, or (else) you will be late.(快点, 要不然就来不及了。) / However I cook eggs, the child still refuses to eat them.(不管我用什么方法煮鸡蛋,小孩还是不肯吃。)

从句按其在复合句中的作用,分为名词性从句（同位语从句、主语从句、表语从句、宾语从句）、定语从句和状语从句等。（参见以下各条）

在句子中作连系动词的表语的从句,它位于主句中的系动词之后。

例如：That is why he did not come to school yesterday. / It is because you are so clever.

在句子中作及物动词或介词的宾语。

①基本形式：（主句+）连词+从句主语+从句谓语+……

② 关于宾语从句连词的选择：

若从句来源于一个陈述句,那么,连词用that,在口语中that可以省略;

若从句来源于一个一般疑问句,连词则用if 或whether;

若从句来源于一个特殊疑问句,则连词就是疑问词(如what,who,where,when等)

例如：They believe that the computer will finally take the place of human beings.(他们相信计算机终将代替人类。) (从句本来就是陈述句)/ I wonder whether I should say something for him to the headmaster. (我不知道是不是该为他在校长跟前说点什么。) (从句来源于一般问句Shall I say something for him to the headmaster?) / He asked me where he could get such medicine. (他问我在哪儿能搞到那样的药。) (从句来源于特殊问句Where can he get such medicine? )

③宾语从句的时态问题：如果主句是现在时,从句则用现在某一时态,甚至可以用过去时；

如主句是过去时,从句则相应地使用过去某一时态,遇到客观真理时仍然用现在时。如：I think I will do better in English this term. (我想本学期我的英语会学得好点。) / The teacher asked the boy if the earth is round. (老师问那个男孩地球是不是圆的。)

④ 下列结构后面的从句一般也作为宾语从句看待：

be sorry / afraid / sure / glad +that从句,如：I’m sorry I’m late. (对不起,我迟到了。) / I’m afraid he isn’t in at the moment. (恐怕他此刻不在家。)

在复合句中作状语,其位置可以在主句前或主句后。状语从句可以分为时间、地点、原因、目的、结果、比较、让步、条件等几种。状语从句由从属连接词引导。

通常由when ,as ,while, after, before, as soon as ,since, till (until), whenever 等引导。时间状语从句一般放在句首或句尾,特别注意,时间状语从句不允许使用将来时,而应该用现在时替代。如： When you finish the work, you may go out to play with Sam. (你完成工作就可以出去和Sam一起玩了。) / I won’t leave until Mum comes back. (妈妈回来了我才会走。)

通常由 where, wherever等引导。如：Go back where you came from! (哪里来还滚到哪里去！) / I will never forget to catch the thief who stole my necklace wherever he may be. (我永远也不会忘记去抓住那个偷我项链的贼,无论他会在哪里。)

通常由because, since, as等引导,一般放在句首或句尾。如：He went abroad because his father had found a good university for him. (他出国了,因为他父亲给他找了一所好大学。)

通常由so that..., so......, in order that... 等引导,往往放在句尾,从句中通常含有can / could / may / might等情态动词。如：He got up earlier so that he could catch the first train. (他起身更早为的是赶上第一班车。)

通常由 so that..., so...that... 等引导,放在句尾。结果状语从句一般表示已经发生的事情,故多为过去时态。如：He lost so many bikes that he decided never to buy a new one. (他丢了那么多辆自行车,他决定再也不买新车了。)

通常由as, than, as (so)...as等引导,一般省略从句的谓语部分,只剩下名词或代词(用主格或宾格均可)。如：Jane is much taller than I/me. (Jane比我高多了。) / I don’t have as many books as you (do). (我书没有你多)

通常由though (although), as, even if( even though), however, whatever等引导。如：Even if you pay the debt(债务) for me, I will not thank you because it has nothing to do with me. (即使你替我还了债我也不会感谢你,因为它与我毫无关系。) / He wears aT-shirtthough it is very cold. (他穿了一件T恤衫,尽管天很冷。)

通常由if, unless, as long as等引导,条件状语从句一般放在句首或句尾,特别注意,时间状语从句不允许使用将来时,而应该用现在时替代。You will certainly fail in the coming final exams unless you work much harder. (即将来到的期末考试你肯定考不及格,除非你更用功。) / If it doesn’ t rain tomorrow, we shall go hiking. (如果明天不下雨我们就要去徒步旅行。)

[注意]

1、because与so；(al)though与but；if与so不可以同在一个句子中成对出现。

2、时间、条件、原因,让步状语从句放在句首时需要用逗号与主句隔开。

在复合句中作定语用修饰句子中某一名词或代词的从句叫定语从句。

①定语从句的位置：放在名词或代词的后面。如：The man who has an umbrella in his hand is my uncle. (手上抓了一把雨伞的人是我的叔叔。)

② 语法术语的改变：被修饰的名词或代词称为先行词；引导定语从句的连接词称为关系词,其中that、which、who称为关系代词,where、when、how称为关系副词。

③关系代词或关系副词的作用：

关系代词who、whom 和whose指人,分别在从句中作主语、宾语和定语。which指物，that多指物, 有时也指人,它们在从句中可以作主语或宾语。定语从句中关系代词作从句宾语时可以省略。This is the thief (that/who/whom) we have been looking for these days. (这个就是我们近日一直在找的小偷。) / Please find a room which is big enough for all of us to live in. (请找一间足够大能住下我们全体的房间；。)

关系副词when或where引导定语从句时,它们在从句中分别作时间状语和地点状语。如：This is the room where they had a quarrel a week ago. (这是一个星期前他们吵架的房间。) / I can never forget the day when I first saw you. (我永远不会忘记第一次见到你的日子。)

④限制性定语从句和非限制性定语从句。限制性定语从句在句中不能省略,否则主句意思就不完整。非限制性定语从句和主句之间往往用逗号分开,这种从句是一种附加说明,如果从句子中省去,也不至于影响主句的意思。如：She spent the whole evening talking about her latest book, which none of us had heard of. (她一个晚上都在谈论她最近的书,我们一个人都没有听说过那本书。)

[注解]

1、关系代词只能用that的情况：当先行词是指事物的不定代词(all, anything等),或先行词部分含有最高级,或含有序数词时,不能用其他的关系代词,只能用that. 如：All that Lily told me seems untrue. (Lily告诉我的话似乎不真实。) / Can you give me anything that has no sugar inside? (能不能给我点里面没有糖的东西？) / This is the first two-story bus that runs in our city. (这是第一辆运行于我市的双层公交车。)

2、关系代词在从句中作主语时一般用who(指人)、that(指物),而不用that指人或用which指物。 如：Do you know the woman who is weeping in the corner? (你认识那个在角落里哭泣的女人吗？) / Do not tell me any words that have nothing to do with me. (跟我无关的话,请一个字也不要说。)

3、关系代词作从句的宾语时,常用which或whom,较少使用that或who,而且,关系代词常常省略。如：Tom’s father was the first parent whom our teacher talked to. (Tom的父亲是我们老师第一个谈话的人。)

4、当关系代词紧跟在介词后面时,必须用which或whom,而当介词移到句尾时,则又可以使用that或who. 如：This is the room in which Miss Li once lived. (= This is the room Miss Li once lived in.) (这是李小姐曾经居住过的房间。)

在句子中充当句子主语的从句叫主语从句。位于谓语动词之前。通常由that,whether以及疑问连词引导。一般情况下,常用it替代主语从句,而将主语从句移到句尾。如：When we should start is still a question. (我们该在什么时候开始还是个问题呢。)