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.
我们用情态动词来表述对现在的确定程度，同样，我们也能用它们对过去进行推测。
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+动词过去式（have done, have been, have stolen等）被称为完成时不定式。当我们使用情态动词来讨论现在，后面跟着一个没有to的不定式。当我们使用情态动词来讨论过去时，他们后面跟着完成时不定式。

must + perfect infinitive
must+完成时不定式

We use must + perfect infinitive when we feel sure about something in the past.
当我们对过去某件事比较肯定时，我们使用“must+完成时不定式”

  • 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+完成时不定式

We use might, may or could with the perfect infinitive to say that we think something was possible but we aren’t sure.
我们使用might、may或者could+完成时不定式来表达我们想到可能但不是特别确定的事情。

  • 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 + 完成时不定式

We use can’t + perfect infinitive when we feel sure something didn’t happen in the past.
当感到事情在过去确实没有发生，我们使用can't+完成时不定式。

  • 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.