what do they usually do改为what are they usually do行不,为什么一定是do

英语动词分成三种,行为动词 系动词与情态动词.do是行为动词,are是系动词,一个句子只能有一个动词.What do they do的第一个do是助动词无意义,不在动词范围内.第二个是行为动词do.比如说They do homework,因为do是行为动词,所以提问时要用助动词提问,一般提问变成Do they do homework?特殊疑问词提问就变成what do they usually do.而what are they do是明显的语法错误,因为有两个动词在一个句子里.