原文
Enclose parenthetic expressions between commas.
插入語前后都要有逗號
例：The best way to see a country, unless you are pressed for time, is to travel on foot.
This rule is difficult to apply; it is frequently hard to decide whether a single word, such as however, or a brief phrase, is or is not parenthetic. If the interruption to the flow of the sentence is but slight, the writer may safely omit the commas. But whether the interruption be slight or considerable, he must never omit one comma and leave the other. Such punctuation as
這條規(guī)則應(yīng)用起來有些困難。人們經(jīng)常會難以判斷一個單詞（例如however）或一個短語是不是插入語。如果僅僅是輕微打斷句子，作者可以安全的省略逗號。但無論打斷是輕微的還是明顯的，他都不能省略一個逗號，留下另一個。例如：
Marjorie's husband, Colonel Nelson paid us a visit yesterday,
or
或
My brother you will be pleased to hear, is now in perfect health,
is indefensible.
這樣的句子是站不住腳的。
Non-restrictive relative clauses are, in accordance with this rule, set off by commas.
遵循這一原則，非限定性關(guān)系從句要用2個逗號和主句分開。例如：
The audience, which had at first been indifferent, became more and more interested.
Similar clauses introduced by where and when are similarly punctuated.
用where或when引導的類似從句也要這樣使用標點。例如：
In 1769, when Napoleon was born, Corsica had but recently been acquired by France.
Nether Stowey, where Coleridge wrote The Rime of the Ancient Mariner, is a few miles from Bridgewater.
In these sentences the clauses introduced by which, when, and where are non-restrictive; they do not limit the application of the words on which they depend, but add, parenthetically, statements supplementing those in the principal clauses. Each sentence is a combination of two statments which might have been made independently.
這些句子中，由which、when、where引導的從句是非限定的。它們只對主句進行附帶的補充，并不對先行詞的用法進行限定。上面的每個句子都是2個獨立句子的集合。
The audience was at first indifferent. Later it became more and more interested.
Napoleon was born in 1769. At that time Corsica had but recently been acquired by France.
Coleridge wrote The Rime of the Ancient Mariner at Nether Stowey. Nether Stowey is only a few miles from Bridgewater.
Restrictive relative clauses are not set off by commas.
限定性從句不用逗號和主句分開。例如：
The candidate who best meets these requirements will obtain the place.
In this sentence the relative clause restricts the application of the word candidate to a single person. Unlike those above, the sentence cannot be split into two independent statements.
這句話中，關(guān)系從句將先行詞candidate限制為一個人。和之前的例句不同，這句話無法分成2個獨立的句子。
The abbreviations etc. and jr. are always preceded by a comma, and except at the end of a sentence, followed by one.
縮寫etc.和jr.的前后都要有逗號，只有出現(xiàn)在句尾時后面的逗號才可以省略。
If a parenthetic expression is preceded by a conjunction, place the first comma before the conjunction, not after it.
如果某個連詞出現(xiàn)在插入部分前面，第一個逗號應(yīng)該放在連詞前面，而非連詞后面。例如：
He saw us coming, and unaware that we had learned of his treachery, greeted us with a smile.
注解
破折號（dash）和括號（parentheses）也可用于插入語。區(qū)別在于破折號通常用于語氣、發(fā)音上的轉(zhuǎn)折，例如：
The party lasted—we knew it would!—far longer than planned.
而括號則用于表示主題以外的補充或解釋，例如：
He hopes (as we all do) that the project will be successful.
