Speculation & deduction

Míriam Tomàs

Logical assumptions and deductions

Expressing speculation or deduction

Depending on the modal verb we use, we’ll express different degrees of certainty. To express speculation / deduction, we use: could, may, might, must and can’t.

There are differences between them; depending on the one we use we are more sure or less sure about what we say.

Could; may; might

Must

Can’tPossible actions.

Almost certain actions.

Impossible actions. Always negative.

Logical assumptions and deductions

▪ Must:

▪ is used in affirmative sentences and expresses positive logical beliefs.

▪ To state something about which you are certain that is true.

must

You have been practising all day,

you must be exhausted!

must

Sean and Jennie must be going out again. I’ve seen them talking very lovey-dovey, you know.

Adding to be + an –ing verb, we can speculate about an action in

progress

Might and may

I’m calling Jane but I can’t get through. She might / may be at class.

We don’t know for sure, but there’s a possibility.

Can’t and couldn’t

▪ Can’t and couldn’t are used in negations and express negative logical assumptions. That is to say, you use them when you’re certain that something is not true or real.

Monsters can’t be real!

Can’t and couldn’t

Someone’s been at home, but it couldn’t be James, he was at work.

Should and ought to

Should and ought to are used to describe a situation we expect to happen.

If I post the letter today, it should / ought to arrive on Friday.

Speculation and deduction in the past

Do you remember how to form the past with modal verbs?

These are the modal verbs we need:

▪ Could / couldn’t

▪ Can / can’t

▪ May / might

▪ Must

▪ Should / ought to

You only need to add…

▪ You can’t / couldn’t have spent very long on this essay –you’ve only written 100 words.

▪ Lidia must have done something to her hair; she looks different.

▪ The letter should have arrived by now.

Could / couldn’tCan / can’tMay / mightMustShould / ought to

have Past participle

▪ I’m not sure where Jake has gone today. He said he wanted to

go to the movies, but his brother wanted to go for a beer… He

might have gone with his brother at the end.

▪ (We’re not very sure of this because we have mixed

information, but we are guessing).

We could also say: …he may have gone / could have gone…

▪ John and Sarah must have gone back together; I

saw them holding hands this morning.

▪ (We are almost sure about this, because we have evidence).

▪ My neighbours can’t have gone out! I definitely saw Mrs

Thompson through the window today!

▪ (We are certain that the neighbours were at home, so it is

impossible that they went out)

Adjectives for speculation

▪ He’s bound / sure to be here in a minute. He left an hour ago.

▪ She’s sure / bound to know. She’s an expert on the subject

Be bound + to + infinitiveBe sure + to + infinitive

We are certain that something is true or will

happen

Adjectives for speculation

▪ I think she’s likely / unlikely to agree to our proposal.

▪ It’s likely / unlikely that the government will raise interests this year.

Be likely / unlikely+ to + infinitive

It is likely / unlikely + that + clause

Adverbs for speculation

▪ She’ll definitely pass the exam. She’s worked really hard.

▪ She definitely won’t pass the exam. She hasn’t done any work at all.

▪ He’ll probably be here around 8.00. he usually leaves work at 7.30.

▪ He probably won’t be here until about 8.15. he’s stuck in a traffic jam.