逗号的用法是正确运用标点过程中最复杂，最容易产生误解的问题之一。在一些情形中，存在着被广泛接受的用法。然而在另一些情况下，却有不只一种用法。学生常常认为，担心标点的用法有点可笑：毕竟，法律分析才是最重要的呀。但是，假如一个人申请一份工作或向上级提交书面报告，没有什么会比忽视标点运用基本规则更容易给人留下负面印象的了。毕竟，一位高级律师是决不会愿意去更正一位初级律师的逗号的用法的。 (来源：英语麦当劳 http://www.EnglishCN.com)
Comma usage is one of the most complex, and most misunderstood, questions of proper punctuation. In some cases there are widely accepted rules governing comma usage; in a few cases, there is more than one acceptable approach. Students often think it's silly to worry about things such as punctuation: after all, isn't the legal analysis what really counts? However, when one applies for a job or submits written work to a supervisor, nothing will leave a more negative impression than ignorance of the basic rules of punctuation. After all, the last thing a senior attorney wants to do is correct a junior attorney's comma usage.
When you begin a sentence with a phrase or dependent clause to introduce a subsequent independent clause, separate the clauses with a comma.
Incorrect: After many years as a criminal prosecutor she ascended to the bench.
Correct: After many years as a criminal prosecutor, she ascended to the bench.
Incorrect: Because the witness was unavailable the judge allowed the introduction of the testimony pursuant to an exception to the hearsay rule.
Correct: Because the witness was unavailable, the judge allowed the introduction of the testimony pursuant to an exception to the hearsay rule.
Use commas to set off a nonrestrictive clause in the middle of a sentence, but not to set off a restrictive clause. Nonrestrictive clauses tell you something about the subject of a sentence, but they do not limit, or restrict, the meaning. Restrictive clauses, on the other hand, limit the possible meaning of the subject.
ompare the following examples.
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.
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.
Use two commas to set off an appositive or an aside in the midst of a sentence. An appositive is a word or phrase that describes a noun it follows. An aside tells us something about the noun, but is not essential to defining the noun.
Correct Use with an Appositive:
The police chief, William A. Bendofsky, is an authority on the use of roadblocks to protect neighborhoods from drive-by shootings.
Correct Use with an Aside:
The pretrial phase of the litigation, like all pretrial work, lasted longer than the trial itself.
Use two commas, not one, to set off a nonrestrictive clause in the middle of a sentence.
Incorrect: The city, a polyglot of different races and religions provided many opportunities for cultural exchange.
Correct: The city, a polyglot of different races and religions, provided many opportunities for cultural exchange.
5.在过渡性的副词前加逗号。比较常用的副词有accordingly, furthermore, however, moreover, therefore, 和 thus。
Place a comma after a transitional word that introduces a sentence. The following are examples of commonly used transitional words: accordingly, furthermore, however, moreover, therefore, and thus.
Incorrect: Accordingly he granted the motion to dismiss.
Correct: Accordingly, he granted the motion to dismiss.
Incorrect: Moreover she convinced the judge that her client had been out of state at the time of the burglary.
Correct: Moreover, she convinced the judge that her client had been out of state at the time of the burglary.
When using commas to separate items in a list, place a comma before the conjunction that precedes the last separate item in the list, unless that last item is a compound term. Many people are taught not to place a comma before a conjunction preceding the last item in a list (such as, "red, white and blue"). However, while popular, this approach runs the risk of creating ambiguity in a number of situations. Consider the following.
Incorrect: The car was available in red, white, black and tan, and special-order colors.
上例中，如果车子有四种标准颜色可供选择，则逗号的用法不正确，因为这种用法使人以为车子还有一种复合颜色（black and tan）可供选择。但如果车子只有三种颜色可选，其中一种是复合颜色（black and tan），那么上述用法就是正确的。
If the car is available in four standard colors, then the above usage is incorrect, because it implies that black and tan is one, two-tone color option. However, if there are only three color options, one of which is black and tan, then the above usage is correct.
Correct: The car was available in red, white, black, and tan, and special-order colors.
The use of the comma before the first conjunction in this sentence makes it clear that there are four standard color options, avoiding the ambiguity created in the first example by the absence of a comma before "and tan."
Use a comma to separate two adjectives that modify the same noun, but do not use a comma if the first of two adjectives modifies the second adjective, but not the noun. In considering this choice, ask yourself whether the two adjectives can be reversed. If they can, as in the first example below, separate them with a comma. If they can not, as in the second example below, do not use a comma.
Incorrect: The only approach to the city was by a long old highway.
Correct: The only approach to the city was by a long, old highway.
Incorrect: The suspect drove a light, blue truck.
Correct: The suspect drove a light blue truck.
In the second example, use of a comma would be incorrect, since "light" modifies "blue," and is therefore not part of a series of commas that modify the word "truck." However, on the off chance that the writer intended to write that the truck was light in weight as well as blue in color, then the first use would be correct. Thus, you can change the meaning of a sentence —— sometimes inadvertently —— by your use of commas.
Do not use a comma to replace the word "that."
Incorrect: The court decided, there is no constitutional right to a second appeal, except by means of a habeas petition.
Correct: The court decided that there is no constitutional right to a second appeal, except by means of a habeas petition.
The court decided there is no constitutional right to a second appeal, except by means of a habeas petition.
In the second correct example, removing the word "that" from the sentence is acceptable because its absence does not confuse the reader. However, leave "that" in a sentence if removing it would create some doubt in the reader's mind concerning what the writer meant to convey.
As a general rule, do not use a comma to separate the parts of a double predicate, unless the sentence would be confusing without it, or the second part of the double predicate requires special emphasis. A double predicate exists where the sentence has one subject and two verbs related to that subject. (Please read rule 10 of this section as well.)
Incorrect: The customer finished his meal, and paid the check.
Correct: The customer finished his meal and paid the check.
Correct Use of Comma to Create Emphasis:
The defendant had been employed as a cashier for twenty years, and never once was accused of stealing money from the register.
In the preceding example, the writer creates a greater emphasis on the defendant's innocence by setting off the second part of the double predicate with a comma. Without the comma, the second thought seems like an afterthought.
Correct Use of Comma to Avoid Confusion:
The judge ruled that suppression of the evidence was required because it had been obtained illegally, and ordered the defendant released from prison.
In this example, the writer avoids confusion by using a comma to introduce the second in the series of compound verbs; without the comma, one might not be sure whether the verb "ordered" related to the subject "it" or the subject "the judge." By contrast, in the sentence above about the restaurant customer, the comma is not needed, because it is a simple sentence.
10. 当用连词连接两个独立从句时，在连词前加逗号。连词包括"and," "but," "or," "nor," 和 "yet"。
When joining two independent clauses with a conjunction, place a comma before the conjunction. Conjunctions include the words "and," "but," "or," "nor," and "yet."
Incorrect: The customer ate every piece of the apple pie and the waitress brought another pie.
Correct: The customer ate every piece of the apple pie, and the waitress brought another pie.
In the incorrect example, the reader may at first think the customer ate the pie and the waitress. In the correct example, the comma before the conjunction tells the reader, "Stop, another independent clause with its own subject (the waitress) is about to begin."
Generally, use a comma before "which" but not before "that." 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."
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.
Place commas inside, not outside, quotation marks. Follow this practice whether or not the comma is part of the original quotation. The general rule is that commas and periods should be inside the quotation marks at all times, while all other forms of punctuation, such as question marks, colons, semicolons, and exclamation points, should be outside the quotation marks, unless they were contained in the original quotation.
Incorrect: The court held that "physical injury is not a required element of a sexual harassment claim", and the plaintiff went on to win her case.
Correct: The court held that "physical injury is not a required element of a sexual harassment claim," and the plaintiff went on to win her case.
Use two commas when setting off dates and places. When using a full date (month, date, year) or multi-part geographical designation (such as city and state or city and country), use two commas around the last part of the designation.
Incorrect: On June 28, 1974 Judge Hayes took her seat on the Supreme Court.
Correct: On June 28, 1974, Judge Hayes took her seat on the Supreme Court.
However: In June 1974 Judge Hayes took her seat on the Supreme Court.
Incorrect: Laredo, Texas was host for the 1994 World Rodeo Championship.
Correct: Laredo, Texas, was host for the 1994 World Rodeo Championship.
Incorrect: We arrived in Tokyo, Japan for the meeting of the G-7 leaders.
Correct: We arrived in Tokyo, Japan, for the meeting of the G-7 leaders.
Cross References: Dependent Clauses and Phrases; Clauses —— Restrictive and Nonrestrictive