It is strange that he should have come back so early .

should have done只用于谈论过去情况,主要有两个用法：■用于推测过去已经发生的情况.如：He should have arrived by now. 此时他本该到了.The letter should have arrived by now. 信现在该到了.■用于指本该发生而...但是 It is adj that sb should do sth 不是固定的么z形容词有important strangeunusual ```````是固定的语法啊这个涉及到虚拟语气的知识，正常来说是上面说的那个意思，但是也有个特例，就是“it's strange that sb should have done"表示他（她）竟然干了这种事，有“竟然”的意思。至于你说的还可以换成其他形容词，你能打个比方嘛，我还真没看到过。比如 It is important that students shuold take some exercise呃····这个就是个正常的主语倒置的句子了。因为主语太长了所以把主语倒置变成一个从句。而且这里也没有shoula have done 啊，这里的should 后面是动词原形take，should这里是正常的情态动词。这句话意思是学生们多做点运动是非常重要的.