1）把非谓语动词做伴随状语的部分，用逗号和句子的其他部分分开。
2）非谓语动词中分词的独立结构，用逗号和句子的其他部分分开。
eg.The content of tomato sauce cooked in an iron pot for three hours showed a striking increase,the level going up nearly 30 times.在这里the level going up nearly 30 times做独立结构，用逗号和句子的其他部分分开。
3）分词短语可做非限定性定语，用逗号和句子的其他部分分开。
eg.But the buffalo nickel,produced from 1931to 1938,honored a pair of connected tragedies from the settlement of the American frontier---------.在这里produced from 1931to 1938做非限定性定语，修饰the buffalo nickel，用逗号和句子的其他部分分开。
4）在状语从句中，用逗号把从句和主句分开。
5）把两个用and或but连接的完整句子，用逗号在and或but前分开。
6）把两个或两个以上并列的平行成分，用逗号隔开。
I reach for the light switch,and as it silently lights the scene,I return to bed to observe the patient with an unemotional,medical eye.在这里第一个逗号是and 连接两个完整的句子，第二个逗号是把as引导的时间状语从句的从句和主句分开，第三个逗号是把两个或两个以上并列的平行成分，用逗号隔开。
7）在非限定性定语从句中，用逗号把从句和主句分开。
eg.The only light in her room is coming from a piece of medical equipment,which is flashing its red light sa if in warning.
8）把插入语，用逗号和句子的其他部分分开。
eg.And people who are likely to have low iron should avoid drinking coffee and tea with meals,she says,since substances in these drinks can interfere with iron being absorbed into the body. she says是插入语。
9）把句子的同位语，用逗号和它前面所解释的名词或名词词组分开。
eg.The first and most common is having low iron reserves,a condition that tapically has no symptoms.在这里a condition ---做同位语解释 low iron reserves，用逗号和它前面所解释的名词或名词词组分开。
10）把句子中的独立成分，用逗号和句子的其他部分分开。
以上几种逗号的作用并不是独立的，有时在一句话会用到上面好几种逗号的作用。如下面这个句子：
And there are others who say their suspicions were aroused over time,as the 55-year-old Mr.Williams,who appeared on weekends turned out in fine suits,bought up property after property,providing such a large injection of cash into the village that he single-handedly brought the community back to life.在这里第一个逗号是把asas引导的原因状语从句的从句和主句分开。第二个逗号是who引导的非限定性定语从句中，用逗号把从句和主句分开，这里也可以把who appeared on weekends turned out in fine suits看成插入语，用逗号和句子的其他部分分开。最后一个逗号是把非谓语动词做伴随状语的部分，用逗号和句子的其他部分分开。
以上例句均选自新视野大学英语3中的句子。
恳切希望众位朋友批评指正，使其变得更加完美谢谢！
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