Modals of Deduction and Speculation

Use modals of deduction and speculation when we want to make a guess about something. We choose the verb depending on how sure we are.

Deducing and speculating about the present

Combine modals of deduction and speculation must, might, could, may and then an infinitive verb to deduce and speculate about an action that happens in the present.

modal (present) + infinitive

Below is a sample scenario using modals of deduction and speculation in present. Know the proper modal to use depending on how true you think that an action occurs.

You are waiting for Amanda with another friend,  Danny.
You ask Danny this question: Where is Amanda?
Danny speculates:

She must be on her way here. (fairly sure this is a good guess)

She might arrive soon. (maybe)

She could be lost. (maybe)

She may be in the wrong place. (maybe)

She can’t be at home. (fairly sure this isn’t true)

Note that in this case, the opposite of ‘must’ is ‘can’t’.

Will / won’t

• We use will and won’t when we are very sure.

He will be at school now.

Should / shouldn’t

• We use should and shouldn’t to make an assumption about what is probably true, if everything is as we expect.

They should get there by now.

It shouldn’t take long to drive here.

• This use of should isn’t usually used for negative events. Instead, it’s a better idea to use will.

Correct: The temperature will be too high today.

Incorrect: The temperature should be too high today.

Can

• We use can for something that is generally possible, something we know sometimes happens.

The cost of living can be very high in big cities.

• We do not use can to talk about specific possibilities.

Correct: Mom could be at the supermarket.

Incorrect: Mom can be at the supermarket.

Deducing and speculating about the past

Put together modals of deduction and speculation must, might, could, may, can’t and have and then a past participle verb to deduce and speculate about an action that occurred in the past.

modal (past) + have + past participle

It must have rained last night.

She might / might not have registered for the seminar.

They could have finished the training if not for the technical issues.

You may / may not have done the dishes this morning.

I can’t have wished ill of another person.

Below is a sample scenario using modals of deduction and speculation in the past. Know the proper modal to use.

Roy asked: Where was Sandra last night?
Maria speculated:

Sandra must have forgotten about our appointment.

She might have worked late.

Sandra could have lost track of time.

She may have gotten sick.

She can’t have stayed at home.

Will

• We use will for something in the past that we are very certain happened.

The parcel will have arrived before now.

Should / Shouldn’t

• We use should and shouldn’t have to make an assumption about something that has probably happened, if everything is as we expect.

The plane should have left by now.

We shouldn’t have gone in that direction.

Could

• We can use could + infinitive to talk about a general possibility in the past.

Labor could be low in the sixteenth century.

• This is not used to talk about specific possibilities in the past. Instead, we use could + have + past participle.

He could have been working late.

(not: ‘could be’. As this is a specific possibility, ‘could be’ is present tense)

Modals of Deduction and Speculation Exercise

Answer this exercise on modals of deduction and speculation.

1. Their shop is easy to locate. It’s right around the corner.  She _____ miss it!

a. might

b. can’t 

c. must

2. Chris’s things are still here so he _____ gone home.

a. shouldn’t have

b. might have

c. can’t have

3. What a great show! They _____ go on a world tour.

a. can’t 

b. might

c. must

4. She can’t remember the suspect’s face. But she _____ seen a distinguishing feature.

a. couldn’t have

b. must have

c. may have

5. It _____ be sugar-free. It has ice cream in it.

a. can’t

b. may not

c. must

6. Clara and Laura weren’t replying to my chat messages. Maybe they went to the movies or they _____ gone to a spa.

a. might have

b. couldn’t have

c. must have

7. Anthony is very stressed out lately. I am not sure but he _____ gone on vacation.

a. can’t have

b. might have

c. must have

8. Your sisters tell different versions of what happened. One of them _____ be lying.

a. must

b. may

c. could

Find the answers to this exercise here.

See also: Modals Continuous and Modals: Might, May, Will, Probably.

For additional information on this topic, check out this British Council lesson.

For other English grammar lessons, go to this page.