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.