someone是后面引导的从句先行词不一定只用that，也可用who。
定语从句的先行词为指人的all, one，ones，someone，anyone，everyone，no one等代词时，引导词只能用who。
One who doesn’t work hard will never get happiness.
不努力工作的人，永远得不到幸福。
Do you know anyone who can mend locks?
你认识会修锁的人吗？
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.
有爱你的人。
there is something that you dont know.
有你不知道的事情。
someone
英 [ˈsʌmwʌn] 美 [ˈsʌmwʌn]
pron.某人，重要人物。
Her father was shot by someone trying to rob his small retail store.
她的父亲被企图抢劫他那间小零售店的人开枪打死了。
扩展资料：
限制性定语从句只能用that的几种情况
一、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.
注意：当先行词指人时，偶尔也可以用who
二、当先行词被序数词修饰。
(1) The first
place that they visited inLondonwas the Big Ben.
三、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.
四、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/
五、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?
六、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?
参考资料来源：百度翻译-someone