Question

it's strange that he should have

It is strange that he should have come back so early .
它的形式应该为“should+动词原形”,
所以 应该是“should come”
可为什么是should have come

Answer 1

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.
■用于指本该发生而实际上未发生的情况.如：
You should have told me so before.你早就应该告诉我.
Look at the time!We should have been at the theatre ten minutes ago.瞧都什么时候了!十分钟前我们就该到戏院了.