“Nonrestrictive Adjective Clauses非限定性定语从句”
An adjective clause set off from the main clause by commas is said to be nonrestrictive.
我们把用“逗号”和主句分开的定语从句叫做“非限定性定语从句”。
Here's an example:
Old Professor Legree, who dresses like a teenager, is going through his second childhood.
老教授Legree，常常穿得像个年轻人，正在经历第二个童年。
This who clause is nonrestrictive because the information in the clause doesn't restrict or limit the noun it modifies (Old Professor Legree). The commas signify that the adjective clause provides added, not essential, information.
“非限定性”是说，who引导的从句并不是必须的，只是额外增加的信息。也就是说，这个句子去掉who引导的`从句，并不影响主句的整体意思：
Old Professor Legree is going through his second childhood.
老教授Legree正在经历第二个童年。
“Restrictive Adjective Clauses限制性定语从句”
On the other hand, an adjective clause that is restrictive should not be set off by commas.
然而，限制性定语从句是不能够用逗号和主句隔开的。
An older person who dresses like a teenager is often an object of ridicule.
年龄不小，穿得还像个年轻人的人，通常是遭人嘲笑的对象。
Here, the adjective clause restricts or limits the meaning of the noun it modifies (An older person). A restrictive adjective clause is not set off by commas.
这里，定语从句限定了它修饰的名词，即An older person。主句和从句之间没有逗号。
如果我们把从句去掉，句子就变成：
An older person is often an object of ridicule.
年龄不小的人通常是遭人嘲笑的对象。
可以看出，句子的意思已经发生了严重的改变。“凡是年龄不小的人通常都是遭人嘲笑的对象”和原句的意思相差甚远。
So let's keep in mind two basic rules:
所以，请务必记住：
Nonrestrictive
An adjective clause that can be omitted from a sentence without affecting the basic meaning of the sentence should be set off by commas.
Restrictive
An adjective clause that cannot be omitted from a sentence without affecting the basic meaning of the sentence should not be set off by commas.
再来一个经典的例子：
1. The house, which is blue, is my house.
2. The house that is blue is my house.
句1的场景：咱们俩在路上聊天，我给你指我家，咱俩都知道说的是哪座房子，只不过我加一句which is blue，多说明一下。如果省略为The house is my house，说话双方依然知道谈的是哪座房子，到底是什么颜色并不影响我们双方的认知。
句2的场景：你完全不知道我家长什么样儿。我说The house that is blue is my house.你通过这个信息才判断出了我家是哪一座。如果省略为The house is my house你就完全一头雾水了！
在使用过程中，which可以用在非限定/限定性定语从句中，而that只能用在限定性定语从句中。
体会一下下面两句话中，限定性和非限定性表达的差异：
非限定：The physical bonds included the network of military garrisons, which were stationed in every province, and the network of stone-built roads that linked the provinces with Rome.
限定：Moreover, the tensions that arose within the Roman state produced literary and artistic sensibilities of the highest order.
另：之前我们说，which最好不要用来修饰整个句子。严格上讲确实如此，但语法界也存在一些争议，例如剑桥大学出版社出版的Advanced Grammar in Use中就提到了非限定性从句which修饰整句的情况：
The book won't be published until next year, which is disappointing.
I have to go to hospital on Monday, which means I won't be able to see you.
