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. (来源：http://www.EnglishCN.com)

　　1.

　　当你用一个短语或从属分句开始一个句子，并随后引入一个独立分句时，用逗号分隔。

　　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.

　　2. 用逗号分隔非限制性从句，但不要用逗号分隔非限制性从句。

　　非限制性从句告诉你关于句子主语的某些事情，但并不对其含义进行限定；相反，限制性从句对主语的含义进行限定。

　　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.

　　3.用两个逗号来分隔同位语或插入语。同位语是对它所跟随的名词进行描述的单词或短语，插入语告诉我们关于某个名词的一些事情，但对阐述该名词并不重要。

　　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.

　　4. 用两个逗号，而不是一个，来分隔非限制性从句。

　　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.

　　6. 当用逗号来分隔列举的项目时，在名单的最后一个项目之前前的连词前面加逗号。有许多人并不这样做，虽然这种用法比较常见，但这种方式在某些情况下有可能会产生歧义。