duty这个单词是名词，意思是“责任; 值班; 职责; 任务”等。on duty是“上班；值班；当值”的意思。例句：
He goes on duty at 8 am and comes off duty at 5 pm.
他上午8点上班下午5点下班.
There was only a skeleton staff on duty.
只有一些基干人员在值班。
There are people on duty round the clock.
一天二十四小时都有人值班。
A policeman must not smoke while he is on duty.
警察当班值道勤回时不得抽烟。
The next crew relief comes on duty at 9 o'clock.
下一批换班的员工9点钟接班。
There will only be a skeleton staff on duty over the holiday.
假期将只留必需的少数职员值班。
The police officer is on duty.
警官在执勤。
It was Mrs Dearden's day off, and Paul was on duty in her place.
那天迪尔登夫人休息，保罗替她当班。
Salespeople in this supermarket must wear name badges when on duty.
这家超市的售货员必须挂牌上岗。
Don't swim unless there is a lifeguard on duty.
除非有救生员值班,否则不要游泳。
The rule is that someone must be on duty at all times.
按照规定，任何时候均需有人值班。
