1、表示主动意义的情况: (1)on 的后面接非动作名词或不及物意义的动作名词,如: The student on duty come to the classroom very early this morning. 值日的那个学生今天一早就来到了教室。 The workers were on strike. 工人们在罢工。 (2)on 后面接动名词时,常作状语,相当于 as soon as…引导的时间状语从句,如: On reading the letter(= As soon as he read the letter), he found that a servant of the family in France had been put in prison, though no fault of his own. 一看完信,他就知道他法国家里的一个仆人无辜地被关进了监狱。 On his arrival in Paris(= When he arrived in Paris), he was recognized as a noble and thrown into prison. 他一到达巴黎,立即被人认为是贵族,因而被投进了监狱。(3)on + the + 动词转化来的名词,表示一个正在进行的动作。如: Keep the kettle on the boil.(= boiling) 让壶里的水开着。
2、表示被动意义的情况 on 后面接具有及物意义的动作名词时,通常表示被动意义。如: Color TV sets are on sale here. 这里在出售彩电。 The flowers on show are from the public garden. 正在展出的那些花是从公园里运来的。