英语的系表结构he must be at home是系表结构吗,it must be terrible to be gr

must是情态动词,后跟动词原形be,所以不是主系表的结构
He must be at home他一定在家
It must be trrrible to be grown up长大成人一定很可怕.
如果是：He is at home now.就是主系表的结构