1.句子有误,须改成：
You shouldn't (have asked) him that question.He couldn't answer it.
2.
1）由后句的couldn't answer it可以看出,ask him that question所表示的动作已经发生.
2）而对过去发生的动作的表达,应该用“should + have done sth”结构.
而should do sth一般表示对现在或将来动作的建议.
句中的shouldn't have done sth翻译为“本不应该做某事（但是当时已经做了）”.
3.改正后的句子译为：
你本不该问他那个问题.他回答不了它.