i do not know how to do it、what to do

应该是I don't know what to do,或者是I don't know how to do it.不可以省点后面的it.
学英语没必要非得去死扣语法,一些简单常用的靠得是语感,用语法反而不易理解,尤其是刚开始学的时候.掌握语法等语感积到一定程度再去学,这时语感同语法互补,效果会更好.
祝你学习成绩不断提高.