In the same way that we use modal verbs to say how certain we are about things in the present we can also use them to speculate about the past.
我們用情態(tài)動(dòng)詞來(lái)表述對(duì)現(xiàn)在的確定程度，同樣，我們也能用它們對(duì)過(guò)去進(jìn)行推測(cè)。
Have + past participle?(‘have done’, ‘have been’ have stolen’?etc.) is called the?perfect infinitive. When we use modal verbs to talk about the present they are followed by an infinitive without ‘to’. When we use modal verbs to talk about the past they are followed by a perfect infinitive.
Have+動(dòng)詞過(guò)去式（have done, have been, have stolen等）被稱為完成時(shí)不定式。當(dāng)我們使用情態(tài)動(dòng)詞來(lái)討論現(xiàn)在，后面跟著一個(gè)沒(méi)有to的不定式。當(dāng)我們使用情態(tài)動(dòng)詞來(lái)討論過(guò)去時(shí)，他們后面跟著完成時(shí)不定式。

must + perfect infinitive
must+完成時(shí)不定式

We use?must + perfect infinitive?when we feel sure about something in the past.
當(dāng)我們對(duì)過(guò)去某件事比較肯定時(shí)，我們使用“must+完成時(shí)不定式”

  • You must have been delighted when you heard you’d won the lottery.
  • The thieves must have come in through the window. Look – it’s still open.
  • Oh no! Where’s my car? Someone must have stolen it!

might/may/could + perfect infinitive
might/may/could+完成時(shí)不定式

We use?might,?may?or?could?with the?perfect infinitive?to say that we think something was possible but we aren’t sure.
我們使用might、may或者could+完成時(shí)不定式來(lái)表達(dá)我們想到可能但不是特別確定的事情。

  • The thieves might have escaped by car but we can’t be sure.
  • He should be hour by now. He may have been delayed by a traffic jam or something.
  • I can’t find my purse. I could have left it in the supermarket but I just don’t know.

can’t + perfect infinitive
can't + 完成時(shí)不定式

We use?can’t + perfect infinitive?when we feel sure something didn’t happen in the past.
當(dāng)感到事情在過(guò)去確實(shí)沒(méi)有發(fā)生，我們使用can't+完成時(shí)不定式。

  • I thought I saw John in town this morning but it can’t have been him – he’s in Greece this week.
  • I can’t have left it in the supermarket – I had it on the bus on the way home.
  • You can’t have read the instructions properly. They’re perfectly clear.