on是副词做表语.表示一种延续的状态.意思是进行中,在这里是开着或开始的意思.可表示电器或自来水,煤气等在开着.再如
the lights are on.灯开着呢.
the film had been on for five minutes when I got to the cinema.当我到达电影院时,电影已经开演了五分钟点了.