Modals verbs - Deduction

A: I don’t understand: he MUST be at home: the lights are on, the car’s in the driveway, I can hear music, it’s his day off, it’s way after closing time, and yet, he hasn't answered the door.

B: He can’t have heard the bell.

A: Oh, I haven’t tried the bell. What a good idea.

Statement of form

Talking about the present: modal verb + base form

Talking about the past: modal verb + have + past participle

What students need to know

This isn't talking about facts; it's what's the speaker believes to be true – usually based on evidence of some kind. He or she is either sure something is true (‘must’), or sure something isn’t true (‘can’t’), or not very sure at all (“might” or “could”).

What students struggle with

Ready? Understanding that "must" and "can't" both mean the speaker is sure; grasping that “can” doesn’t come into it at all, whereas “could” is fine; so is “can’t” and “couldn’t” – all of which refer to the present;  "could" means possible but not certain, whereas "couldn't" is strangely, certain. Confused yet? Try being a student.

And that’s not all. Putting together base form + have + past participle to talk about the past is beyond most students. Even if they manage to put all three correct words in the right order, the contraction of base form and have, so essential to sounding natural, is extremely difficult for them to realise.

Generative situations

Deductions about the present. Take in a photo of a friend or colleague and devise a way to reveal it to the class bit by bit. Let the class speculate about him or her following prompts from you: Man or woman? Drive a car? Married? Has a girlfriend? Likes sport? 

The students' default language of speculation at this stage is likely to be "Maybe he..." or "For sure he...". You reformulate their efforts to arrive at some examples of the target language: “It must be a man.” “He might drive a car, I’m not sure.” “He can’t have a girlfriend, he’s too ugly. Then again, they tend to get the beautiful ones, don’t they.” (Just the first part will do here.)

Deductions about the past. Choose some well-known historical mysteries and present them as questions to the class. For example,

Was President Kennedy killed by the CIA?

What was the purpose of the Peru Nazca lines?

Did Atlantis exist?

Who was Jack the Ripper? 

Did French mathematician Fermat have a solution to his famous ‘Last Theorem’?

Why was Stonehenge built?

Let students discuss which ones they know about, and get any theories they may have, but don't let it go on too long.

Then give a sentence about each one, leaving out the modal + have:

He _____________ had a lot of political enemies.

They _____________ designed to be seen by the Gods.

It _____________ been a real city – it was just invented by Plato.

Whoever it was, he _____________ lived in East London.

The best mathematicians of the twentieth century failed, so he _____________ had proof.

It _____________ been a kind of calendar, because it lines up with the sunrise at mid-summer.

Present the students with the missing modal + have, in random order:

might have, can’t have, must have, must have, can’t have, could have

Ask them to use these to complete the gaps in the sentences. Check the answers, then use the resulting sentences to check that the students understand.

Fun practice

Sudoku. Have your students play the numbers game Sudoku. It is easy to understand if students have not seen it before, and involves pure deduction. Luckily, there are usually several graded versions, from easy and medium to difficult, so students can take their pick. Make sure they voice their thoughts throughout:

"That can't be 2; that must be 5 or 6; that could / might be ...".

Late student. Get the students who have turned up on time to speculate about why a late student is late. They can do this at low levels or high. At low levels, expect most sentences to start with “Maybe…”, which is fine. At higher levels you could prompt students to use modals: “He might have ….” “No, he can’t have …., he must have….” Naturally, if and when the student in question does arrive, the other students (not you) can quiz them as to the real reason for their lateness – making them disinclined to be late again!