on duty 的用法 :the students on duty ___(sweep）the floor when I

on duty 的用法 :the students on duty ___(sweep）the floor when I Sweep the floor everyday is our duty. 10.The students on duty ______the window, when duty call l on on duty I was on duty the next Tuesday.这句话用的是什么时态? I aways sweep the floor on Friday.的否定句,一般疑问句,肯否回答, The police are duty on the duty on road.这里为什么用are 而不是用is? Who ___(be）on duty today Li Ming and I ___(be). One evening Mr.Harris was on duty when the phone rang．He wen One night ,when he was on duty in the bank,a robber broke in One evening Mr Harris was on duty when the phone rang.He wen