主语补足语不是表语滴.
能作表语的词有：
名词：He is a student.
代词：It is mine.
数词：He is 20.
副词：He is out.
形容词:He is young.
动名词：His job is teaching.
现在分词：The film is moving.
过去分词：I was moved to tears.
介词短语：He is at school.
能作主语补足语的词有：
名词：He was elected monitor of his class.
代词：That car was considered his.
副词：The thief was caught out there.
形容词：They arrive safe.
现在分词：He was found lying on the ground.
过去分词：The work was left undone.
介词短语：The thief was caught in a log cabine.