It is strage that he should have come back so early.
It is strage that he should have done...中的should是什么使用方法？
It is strange that he should have come back so early.
本句中的情态动词should 为推定式should , 在that从句中表示“居然、居然”如此的感情色彩，表示怀疑，不相信从句是真的，感觉从句所说的事情是不可思议的。传统语法将should的这种使用方法归到虚拟语气。本句也可以不需要should, 而直接用陈述语气动词形式，如此便是平铺直叙事实，没了上述感情色彩。
It is strange that he came/has come back so early.