助动词的共同特性 （1）都可以放到主语前面去，构成疑问句 疑问词+ 助动词+主语+动词原形+其他 what have they said on this project? （2）都可以在后面加上not构成否定句，并与not可构成简略式 do not- don't shall not-shalln't dare not-daren't ought not-oughtn't等 （3）在简略答句或疑问句尾句中，助动词都可以在省去主要动词的情况下独立做谓语 do you smoke?-yes, i do.
