when he _____something useful,he wasted his time drawing little pictures.
问题描述：
when he _____something useful,he wasted his time drawing little pictures.
a.must have done b.shall be doing c.should be doing d.should have been doing
答
D
should是情态动词,表推测.
have been doing是现在完成进行时,表示从过去到现在一直在做某事.
情态动词should与动词原形连用时表示“应该做某事”,与进行时连用时表示“应该正在做某事”,与现在完成时连用时表示“本应该做了某事”（实际上没有做）,与现在完成进行时连用时表示“本应该一直在做某事（实际上不在做）”.
句意：（以前,）当他在本该做有用的事情的时候,他却浪费时间画小人画.
wasted是过去式,排除A、B.
C项：should表推测,应该.he should be doing something useful意思是“他应该在做某件有用的事”,这与后面的事实相矛盾.排除C.