逗号除了在学术写作中用来表示英文句子的分段以外，它也常被用来引用与介绍下一部分的说明。本期电子报为您介绍逗号的引介用法。
在独立子句前的词组或子句后面要加上逗号
从属子句（dependent clause）：包含主语+谓语，但句意不完整，无法独立成句。
独立子句（independent clause）：包含主语+谓语，单独存在时，句意完整。
词组（phrase）：包含主语或谓语其中之一。
引介的类型及范例
1. 以副词为子句，修饰整句
例句：Surprisingly, the roof was still intact.（令人惊讶的是，屋顶仍然完整。）
此句中，“surprisingly” 作为子句来修饰后面的主句：“the roof was still intact”。
2. 从属子句：以从属连接词开头（如 “after、although、as、when、while、until、unless、before、because、if、since” 等字），且未传达完整句意。
例句：If they want to do well on the test, students must spend time studying.（如果他们想要考试考得好，学生们必须花时间读书。）
句中，“if they want to do well on the test” 子句有主语（they）+谓语（want），表示主句达成的条件仅作为从属子句；若单独存在，句意并不完整。
3. 不定式词组：以动词的不定式形式开头（“to”＋动词）。
例句：To stay in shape for competition, athletes must exercise every day.（为了保持竞赛的良好体能，运动员必须每日运动。）
子句以 “to”＋动词（“stay”）开头，不包含主语，揭示主句发生的目的。这里的主句是：“athletes must exercise every day”。
注意以不定式作为主语的句子，如：“To start a new business without doing market research and long-term planning in advance would be foolish”，则无需逗号，因为该句不是引介用法。这里的 “to”＋原型动词引导的是一个完整子句当做主语，谓语应为动词的单数形式。
4. 分词词组：以过去或现在分词开头。
例句：Throwing caution to the wind, she told him she loved him.（她豁出去了，告诉他她爱他。）
以现在分词开头（“throwing”），整段分词词组后加上逗号，后面跟着主句：“she told him she loved him”。
注意不要将分词句构与动名词混为一谈。句子：“Singing and dancing at the same time is hard to do” 无逗号，“singing” 和 “dancing” 则当名词用，作为此句的主语，这就是动名词的用法。
5. 介词词组：以介词开头。
例句：Over the Christmas holiday, I cleaned out my attic.（在圣诞节假期时，我清理了我的阁楼。）
以介词开头（“over”），逗号在介词词组之后跟着主句：“I cleaned out my attic”。
6. 独立词组：有一个名词和修饰语，通常包括一个分词，但并非绝对。
例句：Their tummies satisfied, they crawled into bed.（肚子饱足后，他们爬上床。）
“Their tummies（名词）satisfied （分词作形容词）”，在整个独立子句加逗号，后面跟着主句：“they crawled into bed”。
练习：放进逗号，如果可以说出是什么类型的引介就更好了。
- Although we had reviewed the film twice before we never noticed these details about the shooting.
- To get everything done I had to start earlier than usual.
- Fortunately I was there to catch him.
- By evening we had become impatient.
- On the other hand I do agree it could use some work.
- Dancing to the music I forgot all of my troubles.
- When he says he will do it he means it.
- Dancing around the room to my favorite song always makes me feel better.
- Teeth clenched, eyes closed she braved the tall slide.
- To be the best was all he could think about.
解答：
- Although we had reviewed the film twice before, we never noticed these details about the shooting.（引介类型：从属子句）
- To get everything done, I had to start earlier than usual.（引介类型：不定式词组）
- Fortunately, I was there to catch him.（引介类型：单一副词后作子句）
- By evening(,) we had become impatient.（可将逗号置于by evening之后，但是非必须，因为这是短介词词组）
- On the other hand, I do agree it could use some work.（引介类型：介词词组）
- Dancing to the music, I forgot all of my troubles.（引介类型：分词词组）
- When he says he will do it, he means it.（引介类型：从属子句）
- Dancing around the room to my favorite song always makes me feel better.（非引介用法，因为dancing around the room to my favorite song是句子的主语）
- Teeth clenched, eyes closed, she braved the tall slide.（引介类型：独立子句）
- To be the best was all he could think about.（非引介用法，因为to be the best是句子的主语）
