Question

i don't know why not do it 这句话对不对？

Answer 1

答：好像应该这么理解这个句子：

I don't know why not to do it . 这个句子的宾语是由“疑问词加上动词不定式短语的否定结构( 即 疑问词+not +to do sth.)充当的. 该结构也可以还原为一个宾语从句，即：why he didn't do it.