keep... from... prevent... from... stop... from...
keep... from doing... 意思是“阻止、防止……做某事”,它与prevent... from doing...和stop... from doing...同义。其中,prevent... from doing...和stop... from doing...中的介词from可以省略(stop 和 prevent用于被动语态时,其后的from 不能省略),但keep…from doing...中的from不能省略。如果省略了from, 那么keep... doing...的意思就成为“使……一直做……”。