在并列成分中的最后一个逗号，常被称作the serial comma（连续逗号）或the Oxford comma（牛津逗号）。

在实际使用中，这个逗号到底是否是必要的，是存有争议的。但是，如果不使用这个逗号，有的句子就会产生歧义。如：

My favourite sandwiches are chicken, bacon and ham and cheese.

My favourite sandwiches are chicken, bacon and ham and cheese.

上句中的bacon后面没有逗号，就会产生歧义：到底三明治的馅料中是包含三种还是四种材料就无法确定了，是包括chicken、bacon、ham and cheese三种材料，还是包括chicken、bacon、ham、cheese四种材料呢？

因此，如果在bacon后面加上逗号，意思就明确了：包含三种材料，即ham and cheese是一种材料：

My favourite sandwiches are chicken, bacon, and ham and cheese.

My favourite sandwiches are chicken, bacon, and ham and cheese.

（二）用于直接引语中

当直接引语位于说话者的前面时，引语的句末往往用逗号隔开，且位于引号内。如：

‘I don't agree,' I replied.

‘Here we are,' they said.

‘I don't agree,' I replied.

‘Here we are,' they said.

如果直接引语位于说话者的后面时，逗号用于说话者的动作之后，且位于引号外。如：

Steve replied, 'No problem.’

Steve replied, 'No problem.’

但是，如果直接引语是感叹句或是疑问句时，句末则分别使用感叹号和问好，而不是逗号。如：

‘Stop him!’ she shouted.

‘Did you see that?’ he asked.

‘Stop him!’ she shouted.

‘Did you see that?’ he asked.

如果说话者前后都有引语时，则前面的引语的句末用逗号，且位于引号内。而说话者的动作后面要用逗号，且位于引号外。如：

‘Yes,’ he said, ‘and I always keep my promises.’

‘Thinking back,’ she added, ‘I didn’t expect to win.’

‘Yes,’ he said, ‘and I always keep my promises.’

‘Thinking back,’ she added, ‘I didn’t expect to win.’

（三）用于分隔主从句

用来分隔主句和从句。如：

After we had had lunch, we went back to work.

If he comes, I will tell him the good news.

After we had had lunch, we went back to work.

If he comes, I will tell him the good news.

这种情况下，逗号使用与否区别不大。

但是，对于定语从句，则区别很大。一般来说，限定性定语从句和先行词之间不用逗号，但非限定性定语从句与先行词则必须用逗号隔开。如：

限定性定语从句：Passengers who have young children may board the aircraft first.

非限定性定语从句：Passengers, who have young children, may board the aircraft first.

限定性定语从句：Passengers who have young children may board the aircraft first.

非限定性定语从句：Passengers, who have young children, may board the aircraft first.

第一句的意思是“有小孩的乘客可以先登机”，意思是没有小孩的乘客要后登机。

而第二句的意思是“乘客可以先登机，他们都带有小孩”，意思是所有乘客都有小孩。

（四）用于分隔句子成分

逗号常用来分隔句子中的非主要成分，如插入语、同位语等。如：

Gunpowder is not, of course, a chemical compound.

His latest film, Calypso Dreams, opens next month.

Gunpowder is not, of course, a chemical compound.

His latest film, Calypso Dreams, opens next month.

第一句中的of course是插入语，而第二句中的Calypso Dreams是同位语，对于整个句子来说都不是主要成分，所以用逗号隔开了。

（五）用于句与句之间的连接词之后

逗号常用来分隔句与句之间的连接词，比如however、therefore等。如：

She felt ill. She went to work, however, and tried to concentrate.

He was the only candidate; therefore, he was elected.

She felt ill. She went to work, however, and tried to concentrate.

He was the only candidate; therefore, he was elected.

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