Explanation
Exercises
Speculation & deduction — modals + set phrases
B2 • quick reference

How sure are we?

must ≈ 90–100% true can’t ≈ 0–10% true may / might / could = possible should / ought to = expected

Use mustn’t for prohibition, not for negative deduction. For “maybe not”, use may/might not (not *could not in this meaning).

Present / future

FormMeaningExamples
must + base
must be + -ing		 very sure it’s true / happening now They must be sisters. • Listen—our neighbours must be arguing.
can’t + base
can’t be + -ing		 very sure it’s not true / not happening He can’t be 50. • They can’t be travelling—no buses today.
may / might / could + base
… be + -ing		 it’s possible (we’re not sure) She might be at home. • Tom may be working from home.
should / ought to + base what we expect to happen They should arrive by midnight. • He ought to be back soon.

Past situations

FormMeaningExamples
must have + past participle
must have been + -ing		 very sure it was true / was happening I must have left my wallet in the taxi. • He must have been sleeping when the alarm went off.
can’t / couldn’t have + past participle
can’t have been + -ing		 very sure it didn’t happen / wasn’t true You couldn’t have seen her—she was abroad. • She can’t have been waiting long.
may / might / could have + past participle
may / might have been + -ing		 it’s possible it happened / was happening They might have finished by now. • She didn’t answer—she may have been having a shower.
could have + past participle (special) it was possible, but it didn’t happen He could have played professionally, but he got injured.
should / ought to have + past participle expected to happen (or mild criticism) The parcel should have arrived yesterday. • He ought to have studied more.

Negative possibility: use may/might not (e.g., “She might not know”). Could not expresses inability, not probability.

Word order with definitely / probably

PatternExample
Before main verb; after be / auxiliary You will definitely get the job. • She is definitely the best option.
With negatives They probably won’t find anything. / They will probably not find anything. • She is definitely not seeing anyone.

Set phrases for likelihood

  • bound / sure be bound/sure to + V → almost certain. He’s bound to win.
  • likely / unlikely
    Two patterns:
    It’s (un)likely that they’ll agree. • He’s (un)likely to sign this week.
  • good chance / odds There’s a good chance prices will drop. • The odds are (that) she’ll call.
  • seem / appear / look preparatory it: It seems they’ve postponed it. • It looks as if it might rain.

Mini-checks & common pitfalls

  • Don’t use mustn’t for deduction. Say can’t: He can’t be 50 (not *He mustn’t be 50).
  • May not / might not = a negative possibility. She might not recognise you.
  • Progressive evidence → use be + -ing after the modal. They must be joking. • He may be working.
  • Ought to is a little more formal than should; both work in deduction of expectation.
Exercises:
1
2
3

Speculation & deduction with modals and set phrases

Choose the correct modal verb or expression for speculation and deduction to complete the sentences below.
1
That the right bus — this route doesn’t stop here.
2
Her office lights are on, so she working late again.
3
The system keeps rejecting your password — you it.
4
He the message — there’s nothing in the Sent folder.
5
Given those clouds, it later this afternoon.
6
From the way they’re shaking hands, they’ve reached an agreement.
7
It’s that she’ll get the job; her references are excellent.
8
With numbers like these, we’re hit the target this quarter.
9
Don’t take it personally — they you; they joke like this with everyone.
10
The lights were on all night — she late at the office.

Speculation & deduction with modals and set phrases

Instruction: Choose two correct options in each question.
1
The lights are on — she ____ at home (present deduction).
A.
B.
C.
D.
2
He ____ the culprit — he was abroad all week (strong negative deduction).
A.
B.
C.
D.
3
Her coat is gone; she ____ early (past deduction).
A.
B.
C.
D.
4
I’m not sure which train he took — he ____ the 6:30 or the 7:00 (uncertainty about past choice).
A.
B.
C.
D.
5
With those clouds, it ____ rain soon (future possibility/likelihood).
A.
B.
C.
D.
6
Your phone’s off again; you ____ ignoring me (present deduction / strong hunch).
A.
B.
C.
D.
7
He ____ seen the email; it’s still unread (past negative deduction).
A.
B.
C.
D.
8
That accent — she ____ from Australia (present deduction with degrees of certainty).
A.
B.
C.
D.
9
Given the time, he ____ at the gym now (present assumption based on routine).
A.
B.
C.
D.
10
After so much practice, they ____ win the tournament (strong expectation).
A.
B.
C.
D.

Speculation & deduction with modals and set phrases

Rewrite the sentences using the word in brackets.
1
Original: I am sure he is at home now.
Transform: He at home now. (must)
2
Original: I am certain she is not the author of this text.
Transform: She the author of this text. (can't)
3
Original: It is impossible that they left before six.
Transform: They before six. (can't have)
4
Original: I am sure he forgot to send the file.
Transform: He to send the file. (must have)
5
Original: Perhaps the data is corrupted.
Transform: The data corrupted. (might)
6
Original: I think it will probably rain later.
Transform: It rain later. (likely)
7
Original: I am almost certain they will notice the error.
Transform: They notice the error. (bound)
8
Original: I am sure she is taking a shower now.
Transform: She a shower now. (must)
9
Original: It seems that the printer is broken.
Transform: It the printer is broken. (looks)
10
Original: It is very probable that he will refuse.
Transform: he will refuse. (chances)