what to do高手快来

I don't konw __________ with the problem
空格处是填what to do
为什么不能用how to do
有什么特殊的语法现象吗?
最好再给我一个例句

快来what do you do? what to do next 还是 what to do the next What to do?什么意思 What do you do to keep fit? What do you want to do? what about do还是what about to do? what to do和how to do的区别 请问how to do与what to do的区别 how to do和what to do的区别 what to do... 和 how to do......的区别是什么？