is indefensible.

Non-restrictive relative clauses are, in accordance with this rule, set off by commas. (来源：英语麦当劳－英语学习门户 EnglishCN.com)

The audience, which had at first been indifferent, became more and more interested. (来源：英语麦当劳 http://www.EnglishCN.com)

Similar clauses introduced by where and when are similarly punctuated. (来源：英语分类信息 http://fl.englishcn.com)

In 1769, when Napoleon was born, Corsica had but recently been acquired by France.

(来源：英语资料下载 http://download.englishcn.com)

Nether Stowey, where Coleridge wrote The Rime of the Ancient Mariner, is a few miles from Bridgewater. (来源：英语交友 http://friends.englishcn.com)

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. (来源：英语e问e答 http://ask.englishcn.com)

The audience was at first indifferent. Later it became more and more interested. (来源：英语分类信息 http://fl.englishcn.com)

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.

(来源：英语博客 http://space.englishcn.com)

Restrictive relative clauses are not set off by commas. (来源：英语电影下载 http://bt.englishcn.com)

The candidate who best meets these requirements will obtain the place. (来源：英语麦当劳－英语杂志 EnglishCN.com)

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. (来源：英语资料下载 http://download.englishcn.com)

The abbreviations etc. and jr. are always preceded by a comma, and except at the end of a sentence, followed by one. (来源：英语麦当劳－英语杂志 EnglishCN.com)

Similar in principle to the enclosing of parenthetic expressions between commas is the setting off by commas of phrases or dependent clauses preceding or following the main clause of a sentence. The sentences quoted in this section and under Rules 4, 5, 6, 7, 16, and 18 should afford sufficient guidance.

If a parenthetic expression is preceded by a conjunction, place the first comma before the conjunction, not after it. (来源：英语交友 http://friends.englishcn.com)

He saw us coming, and unaware that we had learned of his treachery, greeted us with a smile. (来源：英语交友 http://friends.englishcn.com)

4. Place a comma before and or but introducing an independent clause. (来源：专业英语学习网站 http://www.EnglishCN.com)

The early records of the city have disappeared, and the story of its first years can no longer be reconstructed.

(来源：EnglishCN英语问答中心[e问e答])

The situation is perilous, but there is still one chance of escape.

Sentences of this type, isolated from their context, may seem to be in need of rewriting. As they make complete sense when the comma is reached, the second clause has the appearance of an after-thought. Further, and, is the least specific of connectives. Used between independent clauses, it indicates only that a relation exists between them without defining that relation. In the example above, the relation is that of cause and result. The two sentences might be rewritten: (来源：英语电影下载 http://bt.englishcn.com)

As the early records of the city have disappeared, the story of its first years can no longer be reconstructed.

(来源：英语麦当劳－英语快餐EnglishCN.com)

Although the situation is perilous, there is still one chance of escape. (来源：英语美食指南 http://food.englishcn.com)

Or the subordinate clauses might be replaced by phrases:

Owing to the disappearance of the early records of the city, the story of its first years can no longer be reconstructed. (来源：英语麦当劳－英语杂志 EnglishCN.com)

In this perilous situation, there is still one chance of escape. (来源：专业英语学习网站 http://www.EnglishCN.com)

But a writer may err by making his sentences too uniformly compact and periodic, and an occasional loose sentence prevents the style from becoming too formal and gives the reader a certain relief. Consequently, loose sentences of the type first quoted are common in easy, unstudied writing. But a writer should be careful not to construct too many of his sentences after this pattern (see Rule 14). (来源：EnglishCN英语博客基地)

Two-part sentences of which the second member is introduced by as (in the sense of because), for, or, nor, and while (in the sense of and at the same time) likewise require a comma before the conjunction. (来源：英语聊天室 http://chat.EnglishCN.com)

If a dependent clause, or an introductory phrase requiring to be set off by a comma, precedes the second independent clause, no comma is needed after the conjunction.

(来源：英语博客 http://space.englishcn.com)

The situation is perilous, but if we are prepared to act promptly, there is still one chance of escape.

(来源：EnglishCN英语博客基地)

For two-part sentences connected by an adverb, see the next section. (来源：英语分类信息 http://fl.englishcn.com)

5. Do not join independent clauses by a comma. (来源：英语交友 http://friends.englishcn.com)

If two or more clauses, grammatically complete and not joined by a conjunction, are to form a single compound sentence, the proper mark of punctuation is a semicolon. (来源：EnglishCN英语问答中心[e问e答])

Stevenson's romances are entertaining; they are full of exciting adventures. (来源：英语学习门户网站EnglishCN.com)

It is nearly half past five; we cannot reach town before dark.

It is of course equally correct to write the above as two sentences each, replacing the semicolons by periods.

(来源：英语麦当劳－英语学习门户 EnglishCN.com)

Stevenson's romances are entertaining. They are full of exciting adventures.

It is nearly half past five. We cannot reach town before dark. (来源：英语麦当劳 http://www.EnglishCN.com)

If a conjunction is inserted, the proper mark is a comma (Rule 4).

Stevenson's romances are entertaining, for they are full of exciting adventures.