He must have been dreaming为什么加ing

He must have been dreaming为什么加ing I must have been dreaming.(改为否定） 情态动词后面加动词原形,为什么还可以是 he must have been away , you must have been driving at seventy miles an hour为什么要用ing I have been dreaming of He must have been ill,___ he? i must have been dreaming 的语法结构怎么分析呢? You must be dreaming 为什么不说 You are must dreaming? He must have been的用法. he must have been there before----- He must have been asleep at that time,(wasn't) he?为什么是用 wasn i have been teaching since 2006中的teach为什么要加ing?