It is strange that he had made a mistake.奇怪的是他犯了一個(gè)錯(cuò)誤.
這是一個(gè)陳述句,就是陳述一個(gè)事實(shí)：他犯了錯(cuò)誤讓人有點(diǎn)奇怪.
It is strange that he (should) have made a mistake.他居然犯了一個(gè)錯(cuò)誤,真令人奇怪.
這是虛擬語(yǔ)氣,說(shuō)話者是在責(zé)備他本不應(yīng)該犯錯(cuò).對(duì)啊，如果是虛擬語(yǔ)氣的話，那should后應(yīng)該是動(dòng)詞原形make a mistake，為什么是have made a mistake？should have done 這種搭配自身就是虛擬語(yǔ)氣的形式，與時(shí)態(tài)是獨(dú)立的

例如：

I'm really sorry that I should have gone to see you in the hospital，but I had so many things to handle at that moment.
我真的很抱歉，本應(yīng)該到醫(yī)院去看你的，但是我那會(huì)兒有那么多事情要應(yīng)付。

I should have traveled with you last month to Dalian，which is so famous for her beauty,
我本該上月和你去以美麗聞名的大連旅行的。

He should have gone back to his hometown before the Spring Festival.
他本應(yīng)該在春節(jié)前回家鄉(xiāng)的。