Modal verbs
Using modals for ability, permission, obligation, advice and deduction in the present and the past.
Modal verbs (can, could, must, should, may, might and others) add meaning such as ability, permission, obligation, advice and deduction to a main verb. They are followed by the bare infinitive (must go, should leave) and do not change with the subject. The biggest difference for advanced learners is meaning: 'mustn't' means it is forbidden, while 'don't have to' means it is not necessary; 'must be' is a confident present guess, while 'must have been' is a confident guess about the past. To talk about the past we use 'modal + have + past participle', for example: 'You should have told me earlier' (a regret) or 'She can't have left, her coat is still here' (a deduction). For example: 'He isn't answering his phone, so he must be sleeping.'
Key terms
| Modal | Meaning | Example |
|---|---|---|
| can / could | ability | My grandmother can still read without glasses. |
| must / have to | obligation (necessary) | Drivers must stop at a red light. |
| mustn't | prohibition (forbidden) | You mustn't touch that wire. |
| should / ought to | advice | You should see a doctor. |
| must | deduction: sure it is true | The lights are on, so someone must be at home. |
| can't | deduction: sure it is false | That can't be Tom; he's still in Japan. |
| might / may / could | deduction: possible | She might be in the library, but I'm guessing. |
| Modal + have + V3 | Meaning | Example |
|---|---|---|
| must have done | sure it happened | The ground is wet, so it must have rained. |
| can't have done | sure it did NOT happen | She can't have heard the news; she still looks relaxed. |
| might / could have done | possible (uncertain guess) | I might have left my keys at the office, but I'm not certain. |
- 1Sentence: 'The lights are on and I can hear music, so someone ___ be at home.'
- 2Read the evidence: Lights on plus music is strong evidence that the guess is almost certainly true.
- 3Pick the modal: Use 'must' for a confident present deduction. ('can't' would mean you are sure it is false; 'might' would mean only possible.)
- 4Answer: '...so someone must be at home.'
- 1Sentence: 'The ground is wet this morning. It ___ rained during the night.'
- 2Set the time: The rain is finished and in the past, so a bare modal ('must rained') is wrong; the past needs modal + have + past participle.
- 3Choose certainty: Wet ground is strong evidence it happened, so use 'must have' + V3 (rained).
- 4Answer: 'It must have rained during the night.'
Don't confuse the two meanings of 'must'. 'You must wear a uniform' is obligation, but 'Someone must be at home' is a confident deduction. The grammar is the same; the meaning comes from the context.
For a NEGATIVE deduction (sure something is false), use 'can't', not 'mustn't'. 'That can't be Tom' = I'm sure it isn't him; 'mustn't' means 'it is forbidden' and is only for prohibition.
To talk about the past, use modal + have + V3, not a bare modal. Write 'It must have rained' (not 'must rained') and 'You should have told me' (not 'should told me').
'mustn't' and 'don't have to' are not the same. 'You mustn't park here' = it is forbidden; 'You don't have to park here' = it is not necessary. 'Must' also has no past of its own for obligation — use 'had to'.
Rules
- 1Modals are followed by the bare infinitive and never take an -s, -ed or -ing ending: 'She must go' (not 'must to go').
- 2For obligation use 'must' / 'have to'; 'mustn't' = it is forbidden, but 'don't have to' = it is not necessary (no obligation).
- 3For advice use 'should' / 'ought to'; 'had better' gives strong advice or warning about the near future ('You'd better hurry').
- 4For present deduction use 'must' (sure it's true), 'can't' (sure it's false) and 'might / may / could' (possible).
- 5For the past use 'modal + have + past participle': deduction (must / can't / might have done) and regret or criticism (should have done).
Practice
15 easy · 15 medium · 15 hard
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