"why not "后面加do还是to do还是doing？为什么？能不能具体点？

why not do
Why don't 后加人称、谓语
如：
Why don't you take a bus?
Why not take a bus?

do
没为啥...我英语老师整天都说这词组

具体一点应该看你问的这个“why” 是对什么时态进行提问把
比如 对过去式 可以是" why not did" 应该是这样的

do
why not do sth =why don't sb do sth
sb don't do sth你可以理解吧？

to do

why not to do sth

why don't you do sth