(3) In the function of law, “应当” and “必须” differ in their strong and weak legal obligations. Weak behavior requirement looks forward to the inpidual subject doing or not doing some act, but it is not against that the inpidual subject makes the opposite choice. Strong behavior requirement not only looks forward to the inpidual subject doing or not doing some act, but also is strongly opposed to the subject itself making the opposite choice. In the system of social norms, the provisions including “必须” and the provisions including “应当” have different legal effects. A violation of the provisions including “必须” will result in an invalid effect, but a violation of the provisions including “应当” does not necessarily lead to invalid effect. 法律英语中情态动词的语用功能及翻译技巧(3):http://www.youerw.com/yingyu/lunwen_6914.html