你的例句似乎不对. Why not come?=Why don't you come? “疑问词+不定式”结构相当于一个名词从句.一般情况下应该是wh-+不定式 如： I don't know what to do(=what I should do). 从语法上来说,to do可以在前面加上not表示否定. I'm the boss. don't tell me what not to do. 我是老板.不用告诉我什么事不能做. After you know the whole picture you can decide where to save and where not to save. 在你知道了这个之后,你就可以决定哪些是要节省的,哪些是不要的. “疑问词加不定式”肯定有否定用法,不过用得不太多.
