know 后面跟的是一个宾语从句,know (that) handing in homework on time is necessary.
handing in homework 用动名词做主语表示交作业这件事,to do 是交作业这个动作但是宾语从句中that后加to do不是也可以吗?大部分情况是主语或宾语太长，前面用it这个形式主语或形式宾语，后面用to do sth 引导出真正的主语或宾语例如：It is important to hand in homework on time.All the students know it is necessary to hand in homework on time.我的理解是这样的哈。