He is a doctor by profession/calling.

他的职业是医生。

Ex1: He is a doctor.

他是一位医生。

He is a postman. She is a doctor.

他是一名邮递员。她是一名医生。

He is a doctor while I is a worker.

他是一个医生而我是一个工人。

He is a doctor of great repute.

他是个很有声望的医生。

He is a doctor, and he knows his onions.

他是一个经验丰富、 精明能干的医生。

His name tag says he is a doctor.

他的名牌上说他是一名医生。

His name tag? says he is a doctor.

他的名牌上说他是一名医生.

He is a doctor, working in the People's hospital.

他是一位医生，在第一人民医院工作。

He was once a college student, now he is a doctor.

他曾经是位大学生，现在是医生。