Learning objectives
- Use 'can' and 'could' correctly for ability, permission, and possibility
- Use 'may' and 'might' correctly for permission and possibility, with 'might' expressing more uncertainty
- Use 'must' correctly for strong obligation and for logical deduction
- Understand the nuance between 'must' and 'have to' for obligation
- Distinguish 'mustn't' (prohibition) from 'don't have to' (no obligation) — the highest-value distinction in this unit
- Use 'should'/'shouldn't' correctly for advice and recommendation
- Always follow a modal verb with the base form of the verb, never 'to' + infinitive and never with -s
Grammar & vocabulary
Can / could — ability, permission, possibility
'Can' expresses ability or informal permission in the present: "She can speak three languages." "You can sit here if you like." 'Could' is the past of 'can' for ability, and is also used for polite requests and possibility: "When I was younger, I could run much faster." "Could I borrow your pen?" "It could rain later."
May / might — permission, possibility
'May' expresses formal permission or possibility: "You may leave now." "It may rain later." 'Might' also expresses possibility, but generally suggests more uncertainty than 'may': "It might rain later" sounds slightly less certain than "It may rain later."
Must / have to — obligation and deduction
'Must' expresses strong obligation, necessity, or a logical deduction: "You must wear a seatbelt." (obligation) "She must be tired after that trip." (deduction — a strong, logical conclusion, not an obligation). 'Have to' also expresses obligation and is very similar to 'must,' but it often suggests the obligation comes from an outside rule or authority rather than the speaker's own judgment: "I have to renew my passport before the trip" (an external requirement) compared with "I must call my mother tonight" (a personal sense of obligation). In practice, the two are largely interchangeable in most everyday contexts.
Mustn't vs. don't have to — the critical distinction
This is the single most commonly confused point with modal verbs, because the two forms look similar but mean opposite things. 'Mustn't' expresses prohibition — it is forbidden: "You mustn't smoke here." (smoking is not allowed) 'Don't have to' expresses the absence of obligation — it is optional, not required: "You don't have to come if you're busy." (coming is not necessary, but nobody is forbidding it). Mixing these up completely reverses the meaning of the sentence.
Should / shouldn't and will / would
'Should' and 'shouldn't' express advice or recommendation: "You should see a doctor." "You shouldn't stay up so late before an exam." 'Will' expresses the future or a promise/decision made at the moment of speaking: "I will call you tomorrow." 'Would' is used for polite requests and hypothetical situations: "Would you like some tea?" "If I had more time, I would travel more."
Structure: modal + base form
All modal verbs are followed directly by the base form of the verb — never 'to' + infinitive, and never with an -s in the third person singular. "She must go," not "She musts to go." "He can drive," not "He can to drive" or "He cans drive."
Common mistakes
Confusing 'mustn't' with 'don't have to' — these are opposites: 'mustn't' means it is forbidden ('You mustn't smoke here'), while 'don't have to' means it is simply not necessary ('You don't have to come if you're busy'). Using one instead of the other completely reverses the meaning.
Adding 'to' after a modal verb — 'She must to go' is incorrect; modal verbs are always followed directly by the base form: 'She must go.'
Adding -s to the modal verb or the following verb in the third person singular — 'He cans swim' and 'He can swims' are both incorrect; the correct form is 'He can swim,' with no -s anywhere.
Using 'must' for a past obligation — 'must' has no past tense form for obligation; the past of 'must' (obligation) is expressed with 'had to': 'Yesterday, I had to finish my homework,' not 'Yesterday, I must finish my homework.'
Confusing 'may' and 'might' as if they carried exactly the same certainty — both express possibility, but 'might' generally suggests more doubt or a lower likelihood than 'may': 'It may rain later' (fairly possible) vs. 'It might rain later' (less certain, more tentative).
Worked example — Exam day rules
Passage
"On exam day, you must arrive at least fifteen minutes early — this is a strict school rule. You mustn't bring your phone into the exam room; it's strictly forbidden, and anyone caught with one will be disqualified. However, you don't have to bring your own calculator, since the school provides one for everyone. If you feel unwell, you should tell a supervisor immediately. You may leave the room only if a supervisor allows it, and you might be asked to wait outside if there's an issue with your seat number."
Explanation
"must arrive" expresses a strict obligation (school rule). "mustn't bring" expresses prohibition — phones are forbidden. "don't have to bring" expresses the absence of obligation — bringing a calculator is optional since one is provided; note how different this is from "mustn't," even though both refer to bringing something. "should tell" expresses advice. "may leave" expresses formal permission, granted only by the supervisor. "might be asked" expresses a less certain possibility about what could happen.
3 practice exercises
Exercise 1
EasyChoose the correct modal verb for each sentence. 1) 'She ___ speak three languages fluently.' (ability) 2) '___ I borrow your pen, please?' (polite permission) 3) 'You ___ wear a seatbelt in this country — it's the law.' (strong obligation) 4) 'You ___ see a doctor if the pain continues.' (advice)
Show answer
1) can (ability) 2) Could/May (polite permission) 3) must (strong obligation, legal requirement) 4) should (advice).
Exercise 2
MediumChoose the correct option for each sentence: mustn't or don't have to. 1) 'You ___ touch that, it's dangerous.' (it is forbidden) 2) 'You ___ bring your own lunch, the school provides one.' (it is optional) 3) 'Students ___ use their phones during the exam.' (strictly forbidden) 4) 'You ___ finish the whole book tonight, next week is fine too.' (no obligation).
Show answer
1) mustn't (forbidden — dangerous, prohibited) 2) don't have to (optional, not required) 3) mustn't (strictly forbidden) 4) don't have to (no obligation, more time is fine).
Exercise 3
HardFind and correct the modal verb error in each sentence, and briefly explain the correct meaning. 1) 'You mustn't come to the party if you're busy, it's fine.' 2) 'She must finished the report yesterday.' 3) 'He can to drive, he has a licence.'
Show answer
1) 'mustn't' → 'don't have to' — the intended meaning is that coming is optional, not forbidden: 'You don't have to come to the party if you're busy, it's fine.' 2) 'must finished' → 'had to finish' — 'must' has no past form for obligation, so the past is expressed with 'had to': 'She had to finish the report yesterday.' 3) 'can to drive' → 'can drive' — modal verbs are never followed by 'to': 'He can drive, he has a licence.'
3 exercices, c'est un début.
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FAQ
What is the difference between 'mustn't' and 'don't have to'?
'Mustn't' expresses prohibition — it means something is forbidden: 'You mustn't smoke here' (smoking is not allowed). 'Don't have to' expresses the absence of obligation — it means something is optional, not required: 'You don't have to come if you're busy' (coming is not necessary, but not forbidden either). These two are opposites in meaning and are not interchangeable.
What form of the verb follows a modal verb?
Modal verbs (can, could, may, might, must, should, will, would) are always followed by the base form of the verb, with no 'to' and no -s in the third person singular. 'She can speak three languages' is correct; 'She can to speak' and 'She cans speak' are both incorrect.
Can 'must' express something other than obligation?
Yes. Besides strong obligation ('You must wear a seatbelt'), 'must' can also express a logical deduction — a strong conclusion based on evidence: 'She must be tired after that trip' means the speaker is almost certain, based on logic, that she is tired, not that she is obligated to be tired.