非限制性和主句的关系不十分密切，只是对先行词作附加的说明，去掉主句意思仍然清楚。
His brother who lives in London will go to America next week.
他住在伦敦的哥哥（可能还有别的）
His brother，who lives in London ，will go to America next week.
（只有一个哥哥）
2.形式不同：限无逗号，非限用逗号隔开。
3.先行词：限制：一般noun/pron
非限：专有noun或受物主、指示代词修饰的noun或句子
4.关系词不同：that，why不能引导非限
非限：that——which；why——for which
I have told him them the reason, for which I didn’t attend the meeting.
Her son is very smart, which makes her happy.
5.关系词替代情况不同
（1）.限制：whom作宾语时可用who替代whom
非限：whom作宾语时不能用who替代whom，Prep + whom=who
This is the boy whom/who I met at the theatre.
A young man had a new girlfriend, whom he wanted to impress.
（2）.限制：指人——that，who，whom 非限：指人——who，whom
6.关系代词省略情况不同
限制：关系代词作宾语时可省略（prep后不可）
非限：所有关系词不可省略
二.as 和which的选择（非限中）
Same：都可以指代整个句子的内容，非限定语从句位于句末且as 或which作主语、宾语或表语时，常可互换。
He married her, as /which was natural.
He is an old kind fisherman, as /which anybody can see.(宾语)
Different：
(1) 如非限定语从句位于主句之前，只能用as。
As is known to everybody, the moon travels around the earth once every month.
Edison, as is known to all, invented the electric lamp.
(2) as在从句中作主语，后面常+行为v的被动式
Which在从句中作主语，后面常+行为v的主动式
A big earthquake occurred in Sichuan last night, as was reported on TV.
He passed the exam, which made his parents happy.
(3) as引导的从句在意义上不可与主句相悖，而which引导的不受此限制。
Chang’e-1 has been launched successfully, as was expected.
The result of the experiment was very good, which we hadn’t expected.
非限制性定语从句置于句首时，不能用which引导。关系代词as引导非限制性定语从句位置比较灵活，可以放主句前，也可以放主句后。如：
1. As I expected, he didn't believe me. 正如我所预料的，他不相信我。
2. She heard a terrible noise, which brought her heart into her mouth. 她听到一个可怕的声音，这让她把心都提到嗓子眼了
(4) as常用在一些固定结构中
As we know/ as is known to all/ as we all can see
As is often the case 情况常常是这样
As might be imagined 可以想象得到
As might be expected 正如所预料的那样
As has been pointed out 正如所指出的那样
As has been said before/above 正如前文所述
As （it）often happens 像往常一样
She has been absent again, as （it）often happens.
（5）当which在从句中指代的是先行词本身（一个先行词）而不是主句时，不可用as代替which The dog, which we saw in the street yesterday, is Mary’s.
附加
先行词one/ ones/ anyone/those/he 后关系代词用who不用that。
One who is often late should be punished.
The ones who are
Anyone who is
Those who are
He who is
限制性定语从句和非限制性定语从句的区别：
1) 定语从句有限制性和非限制性两种。限制性定语从句是先行词不可缺少的部分，去掉它主句意思往往不明确；非限制性定语从句是先行词的附加说明，去掉了也不会影响主句的意思，它与主句之间通常用逗号分开，例如：
This is the house which we bought last month.这是我们上个月买的那幢房子。（限制性）
The house, which we bought last month, is very nice.这幢房子很漂亮，是我们上个月买的。(非限) 2) 当先行词是专有名词或物主代词和指示代词所修饰时，其后的定语从句通常是非限制性的，例如：Charles Smith, who was my former teacher, retired last year. 查理·史密斯去年退休了，他曾经是我的老师。
My house, which I bought last year, has got a lovely garden.我去年买的的那幢房子带着个漂亮的花园。
This novel, which I have read three times, is very touching. 这本小说很动人，我已经读了三遍。
3) 非限制性定语从句还能将整个主句作为先行词, 对其进行修饰, 这时从句谓语动词要用第三人称单数，例如：
He seems not to have grasped what I meant, which greatly upsets me. 他似乎没抓住我的意思，这使我心烦。
Liquid water changes to vapor, which is called evaporation. 液态水变为蒸汽，这就叫做蒸发。
说明：关系代词that和关系副词why不能引导非限制性定语从句。
非限制性定语从句不能用why引导。要用for which代替why。如：
1. I had told them the reason, for which I didn't attend the meeting. 我已经把理由告诉了他们，为此我没有去开会。
2. I had told them the reason why I didn't attend the meeting. 我告诉了他们我不去开会的理由。(限制性定语从句“the ”是常见搭配。)
在非限制性定语从句中，指人的关系代词作宾语时，只能用宾格whom; 不能用who替换，也不能省略。如：
1. Do you know Tom, whom we talked about? 你认识汤姆吗? 我们谈到过他。
2. This book, which you can get at any bookshop, will give you all the information you need. 这本书会给你提供所有你需要的资料，它在任何一家书店都能买到。(非限制性定语从句中引导词在从句中作宾语，无论指人还是指物都不能省略。)
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