The only thing I can do is (to) do it alone.为什么不能要括号中的to?

问题描述:

The only thing I can do is (to) do it alone.为什么不能要括号中的to?
We have no choice but (swimming) across the river.为什么要用原型而不能用ing形式?
I'm looking for something which to clean the curtain.哪里有错?为什么要这么改?

1.前有do 后无 to 2.have no choice but to do sth 是固定词组 3.with which 介词加关系代词,which 指代sth