Question

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 呢？

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。