分四类.1：助动词后do does did.2：情态动词should can may might must 等 3：固体词组如be supposed to ,can not wait to ,4,to do的形式(区别于介词)5,少数直接加原型的动词如help do sth
分四类.1：助动词后do does did.2：情态动词should can may might must 等 3：固体词组如be supposed to ,can not wait to ,4,to do的形式(区别于介词)5,少数直接加原型的动词如help do sth