I don't know when to leave为什么when后面要加to

I don't know when to leave为什么when后面要加to
I don't know when to leave为什么when后面要加to

I don't know when to leave为什么when后面要加to
怎么问起语法知识到音乐这里来了,这个应该是to 的助动词用法,跟疑问代词连接在一起可以构成宾语,不然不能做宾语,好久没念书了,不知道有没还给老师!＾ ＾!