主语 谓语 主语补足语
He died young.(young是补充说明主语he的一些情况的.)
John was cast as Halmet（as Halmet也是补充说明主语John情况的.）
He was selected as a president.
I work as a teacher.
在前上面的两个句子中,young as Halmet都是可以省略的,只是省略以后,我们就少了解了一些关于主语的信息,例如：
He died.
John was cast.
一般充当主语补足语的都是名词、形容词、或者名词词组、形容词词组等,不能用动词原形.