(To keep)our classroom clean.为什么keep前面要加to？

I'm sorry to keep.为什么keep前面加to It is our duty to keep our classroom clean and tidy.中的It《our It is our duty to keep our classroom clean and tidy.中 our du It is our duty to keep our classroom clean and tidy.句中的 clea 一To keep the classroom clean,our class is divided into six g It's on duty to keep our classroom clean 改为同义句 英语翻译How to keep our classroom clean and tidy first,we should It's n_____for us to keep our classroom clean every day. We should keep our classroom _______(clean). 英语：keep用法1.We must keep the classroom .A.clean B.to clean C. It is our duty to keep our classroom clean and tidy.这句话谁做主语谁 It is our duty to keep our school clean keep 后不是接动名词吗为什么是cle