Modals – deduction past情態動詞——推論（過去時）
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.
我們用情態動詞來表述對現在的確定程度，同樣，我們也能用它們對過去進行推測。
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+動詞過去式（have done, have been, have stolen等）被稱爲完成時不定式。當我們使用情態動詞來討論現在，後面跟着一個沒有to的不定式。當我們使用情態動詞來討論過去時，他們後面跟着完成時不定式。
must + perfect infinitive must+完成時不定式
We use must + perfect infinitive when we feel sure about something in the past.
當我們對過去某件事比較肯定時，我們使用“must+完成時不定式”
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+完成時不定式來表達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 + 完成時不定式
We use can’t + perfect infinitive when we feel sure something didn’t happen in the past.
當感到事情在過去確實沒有發生，我們使用can't+完成時不定式。
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.
