表语从句除可用that,what,when,why,whether,how等引导外，还可由because,as if(though)等引导 。that常可省略 。
如主句主语为reason，只能用that引导表语从句，不可用because.Perhaps the most important thing to remember is that there is no one common type of life in America.The reason why so many people died there is that there were not enough food supplies.It looks as if successful international cultural communication will make the world smaller.4.同位语从句同位语从句用于对前面出现的名词作进一步说明，一般用连词that引导，由于先行名词的意义不同，也可用whether,who,when,where,what,why,how等引导 。常见的先行名词有fact,idea,belief,news,hope,conclusion,evidence,suggestion,order,problem,report,decision.有时由于谓语较短，将同位语从句位于谓语之后 。
She finally made the decision that she would join the fashion show.I had no idea how many books I could borrow at a time.The news came that their team had won the championship.5.定语从句定语从句所修饰的先行词可以是名词或代词，也可以是一个句子 。定语从句通常位于先行词之后，由关系代词或关系副词引导 。
*限制性定语从句限制性定语从句修饰先行词，对先行词起修饰作用，紧接先行词之后，无逗号，若省去，原句意思不完整 。引导定语从句的关系代词有who,whom,whose,which,that等 。
who,whom,whose用于指人，whose有时也可指物，相当于of which;which用于指物；that既可指人也可指物，但只用于限制性定语从句中 。关系代词除了引导定语从句，替代先行词外，还在从句中担任主语、宾语、定语等 。
The computers and cables which make up the Internet are owned by people and organizations.Those who live alone or who are sick may have trouble in getting close to other people.The girl whose parents died in an accident is living with her grandmother.1）当先行词是all,anything,everything,something,nothing等不定代词或先行词前有first,last,any,few,much,some,no,only以及形容词最高级修饰时，只能用关系代词that引导从句 。That is all that I've heard from him.He's the first person that I'm going to interview this afternoon.2）关系代词的省略在从句中作宾语的关系代词常可省略 。
关系代词紧跟介词，作介词宾语时不可用that，只可用which或whom引导从句，并且不可省略，但当介词位于宾语从句句末时，作为介词宾语的关系代词仍可用that，也可省略 。This is one of those things with which we have to put up.This is one of those things (which\that) we have to put up with.3）引导定语从句的关系副词有when,where,why等 。
关系副词在从句中作状语，意义上相当于一个“介词+which”的结构 。Even in comic books where(=in which) there are no words,the stories are fully expressed through the drawings.No one knows the reason why(=for which) he was so angry that day.*非限制性定语从句非限制性定语从句既可修饰先行词，也可修饰整个主句，起补充说明作用，与主句之间有逗号隔开，若省去，原句意思不受影响 。
不可用that引导非限制性定语从句 。关系词不可省略 。
【怎么写主语从句】Every object has a gravitational pull,which is rather like magnetism.*“介词+which\ 。
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