/ 表示可以选用前后词中的任意一个。
不过此句翻译中的must/should中还是最好只用must,其他可以选用。
这是一个主从复合句,其中
让步状语从句:However hard/ difficult it is
主句:the task given to us must be completed/ fulfilled on time
让步状语从句中,However hard/ difficult it is
= No matter how hard/difficult it is
no matter how = however为从句引导词,it是主语,is是系动词,形容词hard/difficult为表语
主句中,主干是:the task must be completed/fulfilled
主语是the task,谓语是must be completed/fulfilled(含情态动词must的被动语态结构)
而given to us为过去分词短语做后置定语,修饰主语the task;
on time为状语。