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.
不定式结构作为句子成分起从句的作用.

to表将来

这是 疑问代词 + 不定式的用法
如：I don't know what to do. I don't know where to go. I don't know how to do. 等等。

：蝴蝶在云端
说的对