“他居然知道了这件事,真是奇怪”--------的翻译是：
It is strange that he should have known about it.
语法结构：
it is strange that + 句子（谓语动词是：should + 一般式/ 完成式 ）
又如：
1 It is unthinkable that he should stay in the office for the night.
2 It was strange that Mary should have come late to such an important meeting.