Think of it like this. A teacher is a person who teaches. A job is something a person does. "My job is a teacher" is incorrect because a job isn't a person who teaches: YOU are the teacher.
可以这样想。Teacher是指那个教书的人。Job是指一个人做的事情。“My job is a teacher”是不正确的,因为工作并不是一个教书的人:而你是那个教书的人。
You can say "I am a..." to introduce your position, and "My job is to..." to introduce your responsibilities.
你可以用“I am a...”来介绍你的职位,或者"My job is to..." 来介绍你的工作内容。
"My job is to treat patients. I am a doctor."
我的工作是治疗病人。我是医生。
"His job is to teach teenagers. He is a High School teacher."
他的工作是教育青少年。他是一位中学老师。
"Their job is to sell houses. They are Estate Agents."