Source: www.britishcouncil.org

Modals – deduction (present)

We use modal verbs to say how sure we are about something.
1 must
We use must when we feel sure that something is true because there’s very strong evidence.
  * He must live near here because he comes to work on foot. We don’t know where he lives but we’re
sure it’s not far away.
  * Come inside and get warm – you must be freezing out there.
  * You’re a zookeeper? That must be very interesting.

Notice that must is followed by an infinitive without ‘to’.
2 might, may, could
We use might, may or could to say that we think something is possible but we’re not sure.
  * Did you hear that? I think there might be a burglar downstairs. She’s not sure there’s a
burglar but she thinks it’s possible.
  * We’ll try to get there early but we may arrive late if there’s a lot of traffic.
  * Don’t put it up there. It could fall off and hit someone.

Might, may and could are also followed by an infinitive without ‘to’.
3 can’t
We use can’t when we feel sure something is not true.
  * It can’t be a burglar. All the doors and windows are locked. He doesn’t know it’s not a burglar
but he feels sure it’s not.
  * It can’t be far away now. We’ve been driving for hours. Where’s the map?
  * Really? He has to work on Christmas Day? He can’t feel very happy about that.

Like the other verbs, can’t is followed by an infinitive without ‘to’.


Modals – deduction (past)

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.
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.
must + perfect infinitive
We use must + perfect infinitive when we feel sure about something in the past.
  * 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
We use might, may or could with the perfect infinitive to say that we think something was possible
but we aren’t sure.
  * 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
We use can’t + perfect infinitive when we feel sure something didn’t happen in the past.
  * 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.