逗号在句中表示一般性停顿。举例：When he came, I was busy cleaning my room.
1. 并列句中，连词（and, but, or, for, so, nor, yet）前用逗号
He asked the question in a loud voice, but no one answered.
We were tied, and we decided to have a rest.
Come early, or you will miss the show.
It must have rain last night, for the ground is wet.
He is an eccentric, yet you can’t help liking him.
但，如果2个或3个并列从句很简短，意思又紧密相连，它们之间也可不用连词，只用逗号。比如：A memoir is history，it is based on evidence.——E.M.Forster。这种用法是特殊情况，比较少见，通常情况下，逗号不能连接前后完成的句子，必须要有连词。举例如下
The teacher came in, we were talking.错误
When the teacher came in, we were talking. 正确
“the teacher came in”和“we were talking”都是主谓宾完整的句子，必须加连词，比如when
When the bell rang, the teacher dismissed the class.（对比The teacher dismissed the class when the bell rang.）
In the ancient palace, they saw the throne where emperors used to sit.
Born and brought up in the Sourth, she is not used to eating wheat.
The audience, interested in the topic, asked the speaker many questions.
On Friday I have an exam. Friday后省略了逗号
On hearing the big noise I knew something terrible happened.
In one corner of the room they found the injured cat.
The teacher dismissed the class when the bell rang.
You will be unable to finish the work in time if you don’t start at once.
He had to leave his home town, where he had lived ever since he was born.
Wang Ling, who is the best singer of the class, will teach us.
My neighbors, John and Kim Sullivan, are both professors at the college.
An old lady, nodding and smilling, invited us in.
The young man (who is) taking with the foreign reporter is the inventor of the machine.
This is the house where the famous poet was born.
The little girl likes to sing, to dance, and to act.
She buys bread, buffer, vegetables, and many other things.
The beautiful, old house had a huge living room.
The guests included the president, the first lady, the vice president and the minister of defence.（四个人都是Guest之列）
He spent the weekend researching his topic, writing his essay and proofreading his final draft. (逗号之后的部分都是在描写he在周末的状态）
The film was, in my opinion, a waste of two hours.
Your work, I am sorry to say, is not satisfactory.
To call a spade a spade, the experiment was a failure.
They have many courses: Chinese, English, Mathematics, History, to name just a few.
Students, therefore, should be allowed to use cell phones in class.
Yes, I think so.
No, it is not very good.
She is the new teacher, isn’t she?
He said, “The Summer Palace is like a fairyland.”
“The Summer Palace is like a fairyland,” He said.
“The Summer Palace,” he said, “ is like a fairyland.”
The delegation will arrive on October 5th, 2000.
She was born on 24th May 1979.
形式主语It的句子，原理上不一定是to do ，只是应用上几乎都是to do