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Enclose parenthetic expressions between commas。插入语前后都要有逗号
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这条规则应用起来有些困难。人们经常会难以判断一个单词(例如however)或一个短语是不是插入语。如果仅仅是轻微打断句子，作者可以安全的省略逗号。但无论打断是轻微的还是明显的，他都不能省略一个逗号，留下另一个。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。这样的句子是站不住脚的。
Non-restrictive relative clauses are, in accordance with this rule, set off by commas。遵循这一原则，非限定性关系从句要用2个逗号和主句分开。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引导的类似从句也要这样使用标点。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。这些句子中，由which、when、where引导的从句是非限定的。它们只对主句进行附带的补充，并不对先行词的用法进行限定。上面的每个句子都是2个独立句子的集合。
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。限定性从句不用逗号和主句分开。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。这句话中，关系从句将先行词candidate限制为一个人。和之前的例句不同，这句话无法分成2个独立的句子。
The abbreviations etc. and jr. are always preceded by a comma, and except at the end of a sentence, followed by one。缩写etc。和jr。的前后都要有逗号，只有出现在句尾时后面的逗号才可以省略。
If a parenthetic expression is preceded by a conjunction, place the first comma before the conjunction, not after it。如果某个连词出现在插入部分前面，第一个逗号应该放在连词前面，而非连词后面。例如：
He saw us coming, and unaware that we had learned of his treachery, greeted us with a smile。
注解
破折号(dash)和括号(parentheses)也可用于插入语。区别在于破折号通常用于语气、发音上的转折，例如：
The party lasted—we knew it would!—far longer than planned。
而括号则用于表示主题以外的补充或解释，例如：
He hopes (as we all do) that the project will be successful。(沪江英语)
