Modal Verbs of Deduction

We use modal verbs of deduction to make guesses or draw conclusions based on the information available.

If we see a man in police uniform, a reasonable deduction is that he is a policeman. If we see a large grey cloud in the sky, a possible guess is that it is going to rain—maybe.

To make a deduction, we need facts and evidence (the uniform, the cloud).

The usual modal verbs that express deduction are:

• must
• might/may, could
• can't

Deduction has degrees of certainty, say from 99% to 30%. Modal verbs of deduction express certainty like this:

• He must be a policeman (because he's wearing a police uniform).
• It might/may/could be going to rain (because there is a large grey cloud in the sky).
• It can't be going to snow (because it never snows in Thailand).

Structure

The basic structure for deduction about the present is:

The basic structure for deduction about the past is:

Modal of deduction: must

We use MUST when we have strong evidence and we are certain that something is true.

• You must be tired after such a long drive.
• She must be the new boss. I saw her in the boardroom.
  
• They must have been shopping. Look at all those bags!
• This window's smashed. The thieves must have climbed in here.

Modal of deduction: might/may, could

We use MIGHT/MAY when we have some evidence and we think something is possible but we’re not certain. Note that the possibility can also be negative — might not, may not.

• Suzie's looking for work. She might be interested in this job offer.
• It's very early. The car might not be ready.
• That may be the phone call I'm expecting.
• She's trying to rest. She may not want any visitors.
• Where are my keys? You might have left them in the shop.
• Joe's a bit deaf. He might not have heard the phone.
• Matt's not in his office. He may have left early.
• It's only 5pm. She may not have finished work yet.

And we use COULD when we're a bit less certain.

• I haven't seen Tim for ages. He could be on holiday.
• The kids aren't home yet. They could have missed the bus.

Modal of deduction: can't

We use CAN'T (CANNOT) when we have strong evidence and we are certain that something is NOT true.

• That can't be the President. He hasn't got any bodyguards.
• She can't be in London yet. It takes three hours at least.
• He can't have heard the phone otherwise he'd have answered it.
• It can't have been the fish that made him sick. He didn't eat any.

Note that these modal verbs can have other uses too. In some cases, they may be used to express a suggestion or a recommendation. For example, "You could try calling him to see if he's available."