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.

本句中的情態(tài)動詞should 為推定式should , 在that從句中表示“居然、居然”如此的感情色彩，表示懷疑，不相信從句是真的，感覺從句所說的事情是不可思議的。傳統(tǒng)語法將should的這種使用方法歸到虛擬語氣。本句也可以不需要should, 而直接用陳述語氣動詞形式，如此便是平鋪直敘事實，沒了上述感情色彩。

It is strange that he came/has come back so early.