限制性从句与非限制性从句
1. Restrictive and Nonrestrictive Clauses Defined. Restrictive clauses limit the possible meaning of a preceding subject. Nonrestrictive clauses tell you something about a preceding subject, but they do not limit, or restrict, the meaning of that subject. Compare the following examples.
含义。限制性从句限定前面主句的含义，而非限制性从句仅讲述与前面的主句相关的内容，但并不对前面的主句进行限定。比较下面的例句：
Correct Restrictive Use:
The suspect in the lineup who has red hair committed the crime.
Note how the subject "suspect" in this sentence is restricted in two ways: we know that this suspect is both in the lineup and has red hair. As a result, we know that the other suspects, who are not in the lineup, could not have committed the crime. Moreover, of those suspects in the lineup, we know that the one suspect in the lineup with red hair committed the crime. If there were more than one suspect in the lineup with red hair, the above usage would be incorrect because it implies a different meaning.
请注意：上句中的主语suspect以两种方式进行了限定：在排队和有红色头发。因此，我们可以知道，其它不在队列中的嫌疑犯没有参与犯罪。而且，在嫌疑犯队列中，只有有红色头发的才参与了犯罪。如果队列中不止一人有红色头发，则上述例句的用法是不正确的，因为它存有其它的含义。
Correct Nonrestrictive Use:
The suspect in the lineup, who owns a red car, committed the crime.
In this example, the restrictive clause "in the lineup" tells us that of all possible suspects in the world, the one who committed the crime is in the lineup. However, while the nonrestrictive clause "who owns a red car" tells us something about the suspect, it does not foreclose the possibility that there are several different suspects in the lineup with red cars. The car color may tell us something useful, but it does not restrict us to only one possibility.
在例句中，限制语in the lineup告诉我们，在所有可能的嫌疑犯中，参与犯罪的嫌疑犯在队列中。尽管非限制性从句who owns a red car告诉了我们关于嫌疑犯的若干资料，但是并不能排除在队列中其它嫌疑犯也拥有red car。车身的颜色可能告诉我们一些有用的东西，但无法限定到仅仅一种可能。
2. When choosing between "that" and "which," use "that" to introduce a restrictive clause and "which" to introduce a nonrestrictive clause. Although some writers use "which" to introduce a restrictive clause, the traditional practice is to use "that" to introduce a restrictive clause and "which" to introduce a nonrestrictive clause. When writing a restrictive clause, do not place a comma before "that." When writing a nonrestrictive clause, do place a comma before "which."
尽管有些作者用which来引入限制性从句，但是传统用法还是用that来引入限制性从句，而用which来引入非限制性从句。使用限制性从句时，that 前一定不要用逗号，而使用非限制性从句时，which 前一定要用逗号。
Correct Restrictive Use:
The store honored the complaints that were less than 60 days old.
Correct Nonrestrictive Use:
The store honored the complaints, which were less than 60 days old.
These sentences have different meanings as well as different punctuation. In the restrictive sentence, the store honored only those complaints less than 60 days old, but not those over 60 days old. In the nonrestrictive sentence, the store honored all the complaints, all of which were less than 60 days old.
这两个句子含义不同，所用标点也不同。在限制性从句中，商店只处理不超过60天的投诉；而在非限制性从句中，商店处理了所有的投诉，这些投诉都是60天内发生的。
3. Place proper punctuation around nonrestrictive clauses, but do not place punctuation around restrictive clauses. When a nonrestrictive clause appears in the middle of a sentence, place commas around it. When a nonrestrictive clause appears at the end of a sentence, place a comma before it and a period after it. Do not punctuate restrictive clauses.
在非限制性从句中可以使用标点，但在限制性从句中则不可以使用。当非限制性成分在句子中间出现时，应当在它前面和后面加逗号。当非限制性成分出现在句末时，在它前面加逗号，后面加句号。不要在限制性从句中使用标点。
Correct Punctuation of Nonrestrictive Clause:
The 1964 Ford Mustang, which propelled Lee Iacocca to the top of the automobile industry, is now considered a classic.
Correct Punctuation of Nonrestrictive Clause:
The credit card is in my wallet, which you can find in the kitchen drawer.
Correct Punctuation of Restrictive Clause:
The boat that sailed on October 25 is the one to which we referred in the contract.
