why not后面到底是to do还是do?

why not后面到底是to do还是do? Why not 后面+doing 还是to do? why not后面是doing sth还是 to do sth WHY NOT 后面是接TO DO 还是直接用DO? why not后加do还是to do?why don‘t you 后面呢? why not sb +do还是to do "why not "后面加do还是to do还是doing?为什么?能不能具体点? could not but后面是do还是to do来着 那个语法是why not to do Sth还是why not do Sth 是Why not to do sth还是Why not do sth? why not 后面+let sb do还是to let sb do? why not 是加to do sth. 还是 动词原形