Tom (主语）
is（谓语）
a junior high school student (表语）属于主语补足语的成分
主语补足语,通常位于主语之前或谓语动词之后,当然放在谓语动词被动语态的过去分词之后,
一般表语都属于主语补语.
如i feel nervous.
nervous即是表语也是主语补语.
被动语态时,主语补语不是表语.
如He was caught alive.
alive是主语补语,不是表语
①主语补语位于主语之前,后有逗号
例：Tired and sleepy,I went to bed
②有时位于主语之后,前后皆有逗号
例：Chen,only 1.30 metres tall,won the first prize.
③位于谓语动词之后
例：He died young
He was found died.
④除形容词外,名词、数词、不定式,分词,介词短语,从句也可作主语补足语.例：He came home quite changed.