duty 指按道德和法律的标准，一个人永远要尽的义务，强调自觉性。
obligation 指道义上或法律上对他人的义务，强调强制性。也指因作出承诺而被迫履行的某种义务。
responsibility 指任何义务、职责、责任或职务上所尽的本分，强调对他人的责任。
function 指因职务或职业关系去履行某种职责。
参考例句
1. As a citizen, we have the obligation to protect our environment. 作为一个公民，我们有义务保护我们的环境。
2. I am under obligation both to Greeks and to barbarians, both to the wise and to the foolish. 无论是希腊人或是未开化的人，聪明的人或是愚笨的人，我都欠他们的债。
3. To fulfill this urgent obligation, the perspective needed is not only national, but also global. 为了履行公民义务，这种需要不仅是全国范围的，而且是全球的。
英英解释
n.
1. the social force that binds you to the courses of action demanded by that force
2. the state of being obligated to do or pay something
3. a personal relation in which one is indebted for a service or favor
4. a legal agreement specifying a payment or action and the penalty for failure to comply
