一、定语从句先行词做主语的例子？
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先行词在定语从句做主语，定语从句谓语单复数由先行词决定。这里，根据先行词类别，分别举例如下:
1. 先行词为人→关系代词用who, 不能省略
The boys that are playing football are from America.
People who create computer viruses are called hackers.
The lady who stands behind the counter has bought a book.
The man who I served was wearing a hat.
2.先行词为物→关系代词用that或which，不能省略。
The books which are on the counter are theirs.
The cars which are produced in Hubei province sell very well.
A dictionary is a book which gives the meaning of words.
特别提醒:非限制性定语从句, 关系代词不能用that，只能用which。
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He who desire nothing is truely happy.无所求者真正快乐。he作主语。
二、关系代词that可以在先行词做主语时用吗？
关系代词that引导定语从句,关系代词指代先行词，可以在从句里面做主语和宾语。
如果先行词做主语的时候，也可以用关系代词that.例如，The book that was written by lu Xun is very popular among young students. 这个句子的主句是the book is very popular部分。
The book是主句里面的主语,That引导的关系代词，用来修饰主语the book。
三、先行词是主句的主语吗？
先行词不一定是主句的主语，先行词是被定语从句所修饰的名词、代词，放在定语从句引导词前。
如:
1.The man who is talking with my mother is a teacher.这句话中the man是先行词，在句中作主语，who is talking with my mother是定语从句。
2. This is the book which I am looking for.这句话中的book是先行词,在句中放在is后是表语，which是定语从句的引导词。
四、先行词指人充当主语主语时用什么？
在定语从句当中，如果先行词是指人的名词，并且相信字在从句当中作主语的时候，可以用关系代词who或者that。
例如,I have a good friend who can often help me with my homework.我有一个可以经常在家庭作业方面帮助我的朋友。
She has a sister who/ that is really stubborn.他有一个非常固执的姐姐。
五、先行词作主语是that可以省略吗？
在定语从句当中，如果先行词做主语，然后用关系代词来替代的话，这个that是不可以省略的。
根据语法规则，只有关系代词that在从句中做宾语的时候，才可以省略。
例如，A plane is a machine that can fly.飞机是一个能飞的机器。
关系代词that在从句中做了主语，不能省略。