It was fortunate that we should not have to carry him. Of course, we should arrest him,but as he has a good record,we shall let him off this time.
we should not have to carry him中的should表示过去将来时。
we should arrest him中 should在这里是一个虚拟语气词，对现在情况的虚拟。本句省略了if条件句
as he has a good record,we shall let him off this time.中shall为情态动词，表示 应该
