may have been推测过去可能处于某种状态
may have done 推测过去可能干过.
may have been doing推测过去可能一直在干.
He may have been a teacher as a young man ; he is good at teaching his kids .
He may have lost his job because he can't afford toys for his son now .
He may have been looking for a job because he is out of work now .