主语一般在句首,是动作的执行者.形容词、数词、副词、doing,done ,to do都可以修饰主语
如：
1.The thick book is mine.
2.Three dollars is enough.
3.The man over there is my father.
4.Doing morning exercises is necessary every day.
5.Broken glass is dangerous.
6.The building to be built next year is a new school.