为什么是must be kept而不是must to be kept

为什么是must be kept而不是must to be kept in the school there are _____rules to be kept.为什么是填enough而不是hard? in the school there are _____rules to be kept.为什么是填hard而不是enough? At school there are strict rules————.为什么是to be kept而不是kept / being kept 他们的区别是什么 文中有一句：Our clavichord is kept in the living room.其中be为什么是用is而不是was? 为什么是must be die而不是must die?die不是动词吗?must be die 是正确的，只要有合适的上下文 Accumulated energy under the earth must be released in one form or another.为什么是Accumulated 而不是 Accumulating呢? I promised toget to the cinema before 5,but now it is pouring with rain.they ___for me impatiently.为什么是must be waiting 而不是must have waited it must be in the living room.( 改为反问疑问句 )it must be in the living room,isn't it为什么是isn't it而不是Must't it 为什么是be busy doing而不是be busy to do 为什么是be fast asleep 而不是be fastly asleep? 为什么是He must wait here.而不是He must waits here. 为什么是you must finish doing your homework,而不是you must finish your homework? -What is behind the door? -There must be a dog sitting behind it. sitting behind it,为什么是sitting?而不是sit 或sat? 高二英语5The oil must be used up,() the light went out.为什么是for而不是because和as? Our room must ___ clean.A.keep B.be kept C.to be D.kept the room must be kept wet.变成被动语态! The medicine must be kept away from children翻译