Modals of obligation, advice and possibility
Review modal verbs for obligation, advice, permission, possibility and deduction — must, have to, should, might, can’t — in present and past.
In this lesson you will
- Choose modals for obligation, advice and permission
- Use might/may/could for possibility
- Use must/can’t for deduction, including the past
Learn
Obligation: must, have to (past: had to). No obligation: don’t have to. Prohibition: mustn’t, can’t.
Advice: should, ought to (past criticism: should have + participle: You should have told me).
Possibility: might, may, could: She might be at home. Past: might have + participle: He might have missed the bus.
Deduction (sure): must (sure it’s true) / can’t (sure it’s not true): You must be tired. That can’t be right. Past: must have / can’t have + participle: She must have left early.
| Meaning | Present | Past |
|---|---|---|
| obligation | must / have to | had to |
| advice | should | should have done |
| possibility | might / may / could | might / could have done |
| sure: true | must | must have done |
| sure: not true | can’t | can’t have done |
Examples
- You don’t have to come if you’re busy.
- You should have booked earlier.
- The lights are off — they must have gone out.
- He can’t have finished already; he only started ten minutes ago.
Vocabulary
deduction
a conclusion from evidence
possibility
something that may be true
criticism
saying what was wrong
prohibition
a rule that says no
certain
sure
Practice
It’s free — you ___ pay.
Speaking
Look at five mysterious situations (an empty office, a smashed window, a friend not answering…) and make deductions with must, might and can’t — present and past.
Writing
Write about a mistake someone made (80–100 words) using should have, could have, must have and might have.
Lesson quiz
I ___ wear a uniform at my last job.
Review
Key points
- must/have to, don’t have to, mustn’t.
- should / should have.
- might/may/could (have).
- must/can’t (have) for deduction.
What you learned
- ✓ I can choose the right modal.
- ✓ I can make deductions.
- ✓ I can talk about the past with modals.
Grammar reference: Must, have to, should: obligation and advice, Modals of deduction: must, might, can’t