should have done 的意思剖析和理解？

The man was elderly and infirm. We can understand his taking an evening stroll， but the ground was damp and the night inclement.

Is it natural that he should stand for five or ten minutes， as Dr. Mortimer， with more practical sense than I should have given him credit for， deduced from the cigar ash？

上面不理解为何用should ？是什么使用方法和意思？

（指Is it natural that he should stand... 和...with more practical sense than I should have given him credit for)

1)natural是含有感情的用词吗？像sorry,regret, surprising， 若是的话，这个我就能理解了。翻译为“居然，居然”，含有感情之意，当然

也可以用陈述语气一般过去时stood（无感情之意），我的理解对吗？

2) 但第二个should不可以理解：with more practical sense than I should have given him credit for.

是否这里是should have done -

a. 与过去不符的虚拟语气

b. 对过去状况的可能性推断

我觉得应该是b. 对过去状况的可能性推断，对吗？

这里的should可以换用成would吗？即with more practical sense than I “would have given him credit for.

1 你的理解正确。

2 虚拟语气。should在此为英式英语(用于第一人称主语，这种使用方法目前极少用），美式英语用would. 中国英语教学中虚拟语气基本都是美式的，即不分人称都用would。