why don't you do sth =why not do sth 干吗不做什么事?
功能意思差不多,都是用来表建议的.
注意：不是 Why not doing 而是 Why not do sth?我们老师就是这么讲的啊why don't you do=why not doing但是考试时候又给我打个错号why not 要接动词原形， 接v-ing 是错的。