句子中逗号的作用

Her father is a doctor and her mother is a teacher.
Her father is a doctor,and her mother is a teacher.
请问这两个句子有什么区别？在英语中运用逗号时，我们是否应该注意些什么？

1、and 用于用来连接具有两个相同语法功能的平行要素，而逗号则用于两个以上除最后一个平行要素外的分开，表示语气的短暂停顿。如：

* Her father is a doctor and her mother is a teacher. ——句中只存在两个要素，因此要直接用 and 连接。
* Her father is a doctor, her mother is a teacher and her brother is an engineer. 句中有三个要素，因此前两个用逗号分开，最后一个用 and 连接。

2、在连接的两个平行结构句子的 and 前，在下述情况也可以用逗号：

1）前一个句子较长，说话人需要有较长时间的换气停顿。如：
* Her father is a doctor in the convalescent home of the city, and her mother is a teacher at a middle school. 他父亲是市疗养院的医生，而她母亲是一所中学的教师。

2）后一个句子中有类似汉语顿号停顿的逗号，则需要在 and 前另加一个逗号，如：
* I did not answer, and he, too, kept silent. 我没有回答, (而)他也不做声。

3、如说话人认为两个并非平行结构的句子之间的关系比逗号或 and 连接较为松散，但又比句号连接的更加密切，则需要用分号。 如：

* Because her father is a doctor, he needs to cure the sickness to save the patient; he always works late at night. 因为他父亲是医生，需要救病治人，他总是工作到深夜（汉语直接可以用逗号）。

如果不用分号，则需要用 and 连接——
* Because her father is a doctor, he needs to cure the sickness to save the patient and he always works late at night.