duty表示“职责”时,是可数名词; 表示“义务”时,是不可数名词。作“税”解时,是可数名词,且常用于复数形式。

do duty for
充作; 当…用 serve in place of; act as substitute for
An old wooden box did duty for a table.一个旧的木箱充作桌子用。
A straw mat did duty for a bed.一条草席当床铺用。

do one's duty
尽职,尽本分,尽义务 do things which one must do, because one thinks it right
She was anxious to do her duty.她极力希望尽自己的本分。
Our dad wasn't a particularly loving father but he did his duty by us.我们的爸爸不是特别慈爱的父亲,可他对我们尽到了本分。
I do my duty as a responsible citizen.我尽一个守法公民的义务。

duty bound
责无旁贷,义不容辞 moral〔personal〕 obligation
It's nice for an old woman to have somebody drop in for a chat, but I wouldn't like you to feel duty bound to come every other day.有人进来聊聊,这对于一个老妇人来说是再好不过了。但我不想让你感到有义务每隔一天来一次。
Duty bound to go to see my brother play cricket,I was expecting to be bored but I quite enjoyed it in the end.我出于义务感去看弟弟打板球,我还以为我会厌烦,但到最后我却很喜欢这种运动。

on〔off〕 duty
值〔不值〕班 engaged〔not engaged〕 in one's regular work
Who is on duty today?今天谁值班?
I was off duty yesterday.昨天我不当班。

The committee carried out the duties entrusted to it.委员会履行了赋予它的职责。
You have performed your duty.你已经履行了自己的职责。
I rely on your doing your duty.我相信你会尽职的。
If I do not report it,I shall fail in my duty.如果我不把这事报告,我就失职了。
Don't neglect your duties.不要玩忽职守!
I'll take over your duty.我将接替你的工作。
He was discharged from his duties last year.他去年被解除了职务。
The newly appointed official has assumed the duties of office.新任命的官员已经就职。
I'll enter upon my new duties tomorrow.我明天将就任新职。
The captain of the ship bears a heavy load of duty and anxiety at all times.船长肩负着重任,总是忧心忡忡的。
She'll perform important duties.她将执行重要任务。
As a mother she has various distracting duties at home.作为母亲,她在家中有各种分心劳神的事要做。
After graduation, he began his duty.他毕业后开始服兵役。
He is a man who maintains duty to his parents.他是一个孝顺父母的人。
We must perform our duty to our parents.我们必须尽对父母的义务。
A duty has been imposed on me.有某种义务强加在我身上。
I regard it as a glorious duty.我把它看成光荣的义务。
This is an inalienable duty.这是一种责无旁贷的义务。
It is a legal, moral duty.这是法律上、道德上的义务。
It is my duty to help you.帮助你是我的义务。
New duties will be imposed on wines and spirits.酒类要征收新税。
This article is free of duties.这货物免税。
The article is subject to customs duties.这商品需纳关税。
Our nation levied duties at certain fixed rates upon wine.国家根据固定税率对酒征了税。
▲～+to-v
It is the primary duty of medicine to save life.医学的首要任务是治病救人。

duty, obligation
这两个词都指“义务”, duty多指“永远的义务”,如duty to one's country; obligation指临时性或一次性的义务,如obligation to wipe the car。

［同义词］   commitment, imperative, obligation, responsibility, function, part
［词源］  ＜英国法语duete＜古法语deu(欠,付)