the chair is soft to ---- 为什么要用SIT ON 不是BE SAT ON

The chair is soft to ____ 。【答案】 sit on【解析】如果to后面的谓语动词是自己发出的,采用主动形式；若是他人发出的,采用被动形式。例：Is there any books to be brought?还有要带过来的书吗?Is there anything (to eat) in the fridge?冰箱里还有吃的东西吗