Modal Verbs — English Grammar Exercises
You mustn't park here, but you don't have to pay — two negatives that look alike and mean opposite things.
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How modal verbs work
Can, could, must, should, might, may: no -s, no to after them, no do in questions: She can swim. You must wear a seatbelt. Can you help me?
Have to and be able to work like normal verbs: She has to work. Does she have to work?
can, could, be able to
| example | |
|---|---|
| ability now | My sister can swim. |
| past ability | When I was five, I could read. |
| one past success | The test was hard, but I was able to pass it. |
| future | You'll be able to speak better. |
| request | Could you open the window, please? |
must and have to
Must has only one form — have to replaces it everywhere else: My brother has to wear a suit. I had to work late yesterday. You'll have to buy a new ticket.
mustn't or don't have to?
- mustn't = not allowed: You mustn't use your phone while driving.
- don't have to = not necessary: The museum is free, so you don't have to pay.
Quick test: can you do it if you want? Yes → don't have to. No → mustn't.
should, ought to, had better
should = advice: You should call your mum. ought to = the same, with to. had better = advice with a warning: We'd better leave now, or we'll miss the train. Negative: You'd better not be late.
might, may, could — possibility
Possible, but not certain: It might rain later. I may be late tonight. Certain → will.
can doesn't predict one future event: ❌ It can snow tomorrow. → ✅ It might snow tomorrow.
Common mistakes
| ❌ | ✅ |
|---|---|
| You must to go. | You must go. |
| Do you can help me? | Can you help me? |
| Yesterday I must work late. | Yesterday I had to work late. |
| I will can come. | I will be able to come. |
| You don't have to smoke here. (= forbidden) | You mustn't smoke here. |
| It can rain tomorrow. | It might rain tomorrow. |
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My little sister is only four, but she ___ already swim.
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can
Modal verbs never take -s, even after 'she', and are followed by a verb without 'to': 'she can swim'.
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You must ___ a seatbelt in the car — it's the law.
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wear
After 'must', use the base verb with no 'to': 'must wear'. 'Must to wear' is a very common mistake.
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You look really tired. You ___ go to bed early tonight.
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should
This is advice → 'should' + base verb, with no 'to'. 'Mustn't go to bed early' would make early sleep forbidden, which makes no sense here.
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Take an umbrella — it ___ rain later.
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might
'Might' = it's possible, but not certain. Like all modals, it has no -s and no 'to' after it: 'it might rain'.
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You ___ use your phone while you're driving. It's dangerous and illegal.
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mustn't
Something is not allowed → 'mustn't'. 'Don't have to' means it isn't necessary, which is not the meaning here. 'Don't must' doesn't exist.
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I ___ work late yesterday, so I missed the film.
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had to
'Must' has no past form. For an obligation in the past ('yesterday'), use 'had to'.
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You ___ tell anyone about the party — it's a surprise for Mum!
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mustn't
Telling anyone would spoil the surprise, so it's not allowed → 'mustn't'. 'Don't have to' would mean you can tell people if you want.
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When I was five, I ___ read, but I couldn't write.
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could
'Could' is the past of 'can' for a general ability in the past ('when I was five'). 'Was able' needs 'to': 'was able to read'.
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I think you ___ call your mum. She's worried about you.
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should
'I think you should…' gives advice. Mum is worried, so calling her is a good idea → 'should', not 'shouldn't'. No 'to' after 'should'.
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I'm not sure yet, but I ___ visit my parents this weekend.
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might
'I'm not sure' → it's only possible → 'might'. 'Will' means you're sure, and 'must' means it's necessary — neither matches 'not sure'.
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Tomorrow is Saturday, so I ___ get up early.
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don't have to
There's no work, so getting up early isn't necessary → 'don't have to'. 'Mustn't' would mean getting up early is forbidden.
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My brother ___ wear a suit at work, but I can wear jeans.
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has to
'Have to' works like a normal verb, so it takes -s after 'he/she': 'my brother has to'. 'Must' is never followed by 'to'.
Modal Verbs in English: One Set of Rules, Many Meanings
Modal verbs — can, could, must, should, might, may, will and would — add a speaker's attitude to a verb: whether something is possible, necessary, a good idea or allowed. They are among the most frequent words in English, and large corpus studies of English grammar find them especially dense in conversation, where speakers constantly negotiate plans, permissions and advice. Their grammar is unusually simple: no -s after he, she or it, no to before the next verb, and no do in questions and negatives. That simplicity is exactly what causes errors, because learners apply the rules of ordinary verbs to them and produce she cans, must to go and Do you can help me? The meanings, by contrast, overlap and shift: one modal can carry several meanings, and several modals can share one.
Must and Have To: Close in Meaning, Different in Form
In a positive sentence about the present, You must wear a seatbelt and You have to wear a seatbelt say almost the same thing. The difference that matters for accuracy is not meaning but form. Must exists only in the present, so everywhere else have to takes over: had to in the past, will have to in the future, has to after he and she, and Does she have to…? in questions. Studies of written English across the twentieth century have also tracked a steady decline of must in favour of have to, especially in American English, so learners who default to have to are following the direction the language itself is moving.
Mustn't or Don't Have To: The Biggest Trap
Because must and have to are so close, learners expect their negatives to be close too. They are opposites. Mustn't means something is not allowed: You mustn't use your phone while driving. Don't have to means it is not necessary, and you are free to do it if you want: The museum is free, so you don't have to pay. Confusing the two changes the message completely — You don't have to smoke here grants permission where the speaker meant to forbid. The reliable test is a single question: can you do it if you want? If yes, the answer is don't have to; if no, it is mustn't.
Ability, Advice and Possibility
Can expresses present ability and permission, could its general past form, and be able to covers the tenses can lacks: will be able to for the future, and was able to for a single past success, where could is not used. Should gives advice, ought to means the same with a to, and had better adds a warning: We'd better leave now, or we'll miss the train. For possibility, might, may and could all mean “possible but not certain”, sitting between will and won't on a scale of certainty. Can is not on that scale: It might snow tomorrow, never It can snow tomorrow.
Common Mistakes with Modal Verbs
- ✗ You must to go. → ✓ You must go.
- ✗ Do you can help me? → ✓ Can you help me?
- ✗ Yesterday I must work late. → ✓ Yesterday I had to work late.
- ✗ I will can come. → ✓ I will be able to come.
- ✗ You don't have to smoke here. (= forbidden) → ✓ You mustn't smoke here.
- ✗ It can rain tomorrow. → ✓ It might rain tomorrow.
Frequently Asked Questions
What is the difference between mustn't and don't have to?
'Mustn't' means something is not allowed: 'You mustn't use your phone while driving.' 'Don't have to' means something is not necessary — you can do it if you want, but you don't need to: 'The museum is free, so you don't have to pay.' The positive forms 'must' and 'have to' are very close in meaning, but the negatives are opposites. A quick test: if you can do it when you want, use 'don't have to'; if you can't, use 'mustn't'.
What is the past tense of must?
'Must' has no past form. For an obligation in the past, use 'had to': 'I had to work late yesterday', not 'I must work late yesterday'. In the same way, the future is 'will have to' ('You'll have to buy a new ticket'), because 'will must' is impossible — two modal verbs can't go together.
Can you use 'to' after modal verbs?
No. Modal verbs — can, could, must, should, might, may, will, would — are followed by the base verb with no 'to': 'You must wear a seatbelt', 'You should call her', 'It might rain'. 'Must to' and 'should to' are among the most common mistakes in learner English. The exceptions are 'have to', 'be able to' and 'ought to', which always include 'to'.
What is the difference between could and was able to?
For a general ability in the past, both work: 'When I was five, I could read.' For one specific success on one occasion, use 'was able to' or 'managed to', not 'could': 'The test was hard, but I was able to pass it.' In the negative, 'couldn't' is fine in both cases: 'I couldn't find the hotel.'
Last updated: · Satori Editorial