先行词是不定代词anything, everything, nothing，few, all, none, little, some代词时。
由every, any, all, some, no, little, few, much等修饰时也用that。
someone指人时，后面可以用who。
例：who has a sense of duty won”t do such a thing.
有责任感的人是不会做这种事的。
that的用法
1、that用作指示代词的意思是“那，那个”。可指前面提到的那件事，也可指在空间或时间上较另一事物远的事物或人，还可指要表明的或要提及的事物、想法等。
2、that用作关系代词时，指人或事物，同时又在从句中充当句子成分。that偶尔也可引导非限制性定语从句。
3、that还常用以代替who， whom， which，但that...which中的that用做关系代词的先行词。
4、that用作关系代词时在下列情况下可以省略：
①引导限制性定语从句在从句中用作宾语时。
②用在there be结构前作主语时。
③用作表语时。
④在定语从句与先行词之间有插入语时。
⑤在先行词way， reason， distance， place， office等作方式状语、原因状语或地点状语时。
5、that用作连词时可引导主语从句、宾语从句、表语从句、同位语从句和状语从句。
6、that还常引导间接引语或形容词后面的从句。
7、that在以it作形式主语的句子中或引导宾语从句、状语从句时常可省略。
8、that也可用于表示愿望或遗憾。
扩展资料
词汇解析
someone
英 ['sʌmwʌn]；美 ['sʌmwʌn]
pron. 有人，某人；重要人物
例：Her father was shot by someone trying to rob his small retail shop.
她父亲被某个试图抢劫他的小零售店的人开枪打中了。
例："Before she came around," she says, "I was somebody in this town."
“她来这之前，”她说，“我在这个镇上是个人物。”
难点分析
（一）限制性定语从句只能用that的几种情况
1．当先行词是anything, everything, nothing (something除外), few, all, none, little, some等代词时，或者是由every, any, all, some, no, little, few, much等修饰时
(1) Have you taken
down everything that Mr. Li has said?
(2) There seems to
be nothing that seems impossible for him in the world.
(3) All that can
be done has been done.
(4) There is
little that I can do for you.
注意：当先行词指人时，偶尔也可以用who
(4) Any man
that/.who has a sense of duty won”t do such a thing.
2.当先行词被序数词修饰
(1) The first
place that they visited inLondonwas the Big Ben.
3.当先行词被形容词最高级或比较级修饰时
(1) This is the
best film that I have seen.
4.当先行词被the very, the only修饰时
(1) This is the
very dictionary that I want to buy,
(2) After the fire
in his house, the old car is the only thing that he owned.
当先行词指人时，偶尔也可以用who
(3) Wang Hua is
the only person in our school who will attend the meeting/
5.当先行词前面有who, which等疑问代词时
(1) Who is the man
that is standing there?
(2) Which is the
T-shirt that fits me most?
6.当先行词既有人，也有动物或者物体时
(1) Can you
remember the scientist and his theory that we have learned?
只用which, whom
1. .当先行词有限定性修饰词时
what was the name
of the war in theUSAwhich lasted 5 years?
2. 当先行词是one, ones, anyone,等时用who;
those做先行词时，指人用who, 指物用which
Those who want to
go to the Great Wall sign up here.
He who has never been to the Great Wall is not a true man.
本回答被提问者采纳someone后 that 和who都可以用
但是
如果定语从句缺主语 that就不能用了 因为that不充当成分 只能用who
there is someone who loves you
there is something that you dont know (that可省略)追答
😂好像我错了 定语从句语法好几年没看了
本回答被网友采纳指物的不定代词比如：Somethin，anything等作先行词时，用that引导定语从句，不用which