Must and Can't | Unit 28 | PrepMyEnglish

Unit 28 · Grammar Lesson

Unité 28 · Leçon de Grammaire

Must and Can't
Logical Deduction — Present & Past — Complete Guide Must et Can't
Déduction Logique — Présent & Passé — Guide Complet

Master how to use must and can't to make logical deductions about the present and past — You must be tired / They can't have known / She must have left

Maîtrisez l'utilisation de must et can't pour faire des déductions logiques sur le présent et le passé — You must be tired / They can't have known / She must have left

B1 – B1+ 2 Rules · 3 Exercises · 200-Q Quiz · PDF 2 Règles · 3 Exercices · Quiz 200 Q · PDF 🇬🇧 EN / 🇫🇷 FR

In this unit we look at a very important use of must and can't: making logical deductions. When you look at the evidence around you and draw a conclusion, you use must (= I'm certain it's true) or can't (= I'm certain it's not possible). For the past, we use must have and can't have (or couldn't have).

Dans cette unité, nous étudions une utilisation très importante de must et can't : faire des déductions logiques. Quand vous regardez les preuves autour de vous et tirez une conclusion, vous utilisez must (= je suis certain que c'est vrai) ou can't (= je suis certain que ce n'est pas possible). Pour le passé, on utilise must have et can't have (ou couldn't have).

Must & Can't — Logical Deductions About the Present

Must & Can't — Déductions logiques sur le présent

We use must to say that we believe something is certain — based on evidence or logic. We use can't to say that we believe something is not possible — it contradicts what we know.

On utilise must pour dire que l'on croit que quelque chose est certain — sur la base de preuves ou de la logique. On utilise can't pour dire que l'on croit que quelque chose n'est pas possible — cela contredit ce que l'on sait.

Subject + must + base verb → certain / logical conclusion
Subject + can't + base verb → impossible / contradicts logic
You must be tired. / You can't be hungry already.

Must — Certain (logically true)

Must — Certain (logiquement vrai)

evidence → logical conclusion it IS true

preuve → conclusion logique que c'EST vrai

You've been travelling all day. You must be tired.

'Joe? You must be joking. He never works!'

Louise must get very bored. She does the same thing every day.

My house is near the motorway. It must be very noisy.

They stay at five-star hotels. They must have a lot of money.

Can't — Impossible (logically false)

Can't — Impossible (logiquement faux)

evidence → conclusion it CANNOT be true

preuve → conclusion que ça NE PEUT PAS être vrai

You've just had lunch. You can't be hungry already.

They haven't lived here long. They can't know many people.

He runs marathons every week. He can't be unfit.

She works 80 hours a week. She can't have much free time.

Structure summary:

Résumé de la structure :

Subject	Modal	Verb form	Example
you / she / they	must	be (tired, hungry…)	You must be tired.
you / she / they	must	be + -ing (doing, joking…)	You must be joking.
you / she / they	must	get / know / have…	She must get very bored.
you / she / they	can't	be (hungry, right…)	You can't be hungry.
you / she / they	can't	know / have / live…	They can't know many people.

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Key difference:Différence clé : Must = I'm sure it IS true (positive certainty). Can't = I'm sure it is NOT possible (negative certainty). Both are based on reasoning and evidence — not direct knowledge. Must = je suis sûr que c'EST vrai (certitude positive). Can't = je suis sûr que ce n'est PAS possible (certitude négative). Les deux sont basés sur le raisonnement et les preuves — pas sur une connaissance directe.

Must Have & Can't Have — Deductions About the Past

Must Have & Can't Have — Déductions sur le passé

For deductions about the past, we use must have + past participle and can't have + past participle. You can also use couldn't have instead of can't have.

Pour les déductions sur le passé, on utilise must have + participe passé et can't have + participe passé. On peut aussi utiliser couldn't have à la place de can't have.

Subject + must have + past participle → certain past conclusion
Subject + can't have + past participle → impossible past conclusion
They must have gone out. / She can't have got my message.
(couldn't have = can't have — both are correct)

Must Have — Certain (past)

Must Have — Certain (passé)

the only logical past explanation

la seule explication logique passée

Nobody answered the door. They must have gone out.

I lost a glove. I must have dropped it somewhere.

'We lived near the motorway.' 'It must have been noisy.'

You got here fast. You must have driven quickly.

Can't Have / Couldn't Have — Impossible (past)

Can't Have / Couldn't Have — Impossible (passé)

contradicts what we know about the past

contredit ce que l'on sait du passé

Sarah can't have got my message. (= couldn't have got)

Max can't have been looking where he was going.

The bill can't have been correct — it was far too high.

He couldn't have known — nobody told him.

Structure summary — past deductions:

Résumé de la structure — déductions passées :

Subject	Modal	have	Past participle	Example
I / you / he…	must	have	been / gone / got…	They must have gone out.
I / you / he…	must	have	been + -ing	He must have been sleeping.
I / you / he…	can't	have	been / got / known…	She can't have got my message.
I / you / he…	couldn't	have	been + -ing	He couldn't have been looking.

📌

Can't have = couldn't have:Can't have = couldn't have : For past negative deductions, both can't have and couldn't have are correct and interchangeable. Example: She can't have got my message = She couldn't have got my message. Pour les déductions négatives passées, can't have et couldn't have sont tous les deux corrects et interchangeables. Exemple : She can't have got my message = She couldn't have got my message.

⚠️

Don't confuse with obligation:Ne pas confondre avec l'obligation : Must also means obligation (I must go now). In this unit, must is used for deduction only — saying what you believe to be certain based on evidence. Context makes it clear which meaning is intended. Must exprime aussi l'obligation (I must go now). Dans cette unité, must est utilisé pour la déduction uniquement — dire ce que l'on croit être certain sur la base de preuves.

EX 1

Exercise 1 — Put in must or can't

Exercice 1 — Mettez must ou can't

Choose must or can't for a logical deduction in each sentence.

Choisissez must ou can't pour une déduction logique dans chaque phrase.

You've been travelling all day. You be tired.Tu as voyagé toute la journée. Tu être fatigué.

That restaurant be very good. It's always full of people.Ce restaurant être très bien. Il est toujours plein.

That restaurant be very good. It's always empty.Ce restaurant être très bien. Il est toujours vide.

I'm sure Kate gave me her address. I have it somewhere.Je suis sûr(e) que Kate m'a donné son adresse. Je l'avoir quelque part.

I often see that man in this street. He live near here.Je vois souvent cet homme dans cette rue. Il habiter près d'ici.

It rained every day during their holiday. It have been very nice for them.Il a plu tous les jours pendant leurs vacances. Ça avoir été très agréable pour eux.

Congratulations on passing your exam! You be very pleased.Félicitations pour ton examen ! Tu être très content(e).

This bill be correct. It's much too high.Cette facture être correcte. Elle est beaucoup trop élevée.

You got here very quickly. You have driven very fast.Tu es arrivé(e) très vite. Tu avoir conduit très vite.

10.

Bill and Sue always stay at five-star hotels. They be short of money.Bill et Sue séjournent toujours dans des hôtels cinq étoiles. Ils manquer d'argent.

11.

Karen hasn't left the office yet. She be working late tonight.Karen n'est pas encore partie du bureau. Elle travailler tard ce soir.

Answers:Réponses : 1. must · 2. must · 3. can't · 4. must · 5. must · 6. can't · 7. must · 8. can't · 9. must · 10. can't · 11. must

EX 2

Exercise 2 — Complete with a verb (one or two words)

Exercice 2 — Complétez avec un verbe (un ou deux mots)

Add the correct verb after must or must have.

Ajoutez le verbe correct après must ou must have.

I've lost one of my gloves. I must have it somewhere.J'ai perdu un de mes gants. Je dois l'avoir quelque part.

Their house is very near the motorway. It must very noisy.Leur maison est très proche de l'autoroute. Ça doit très bruyant.

You've lived here a long time. You must everybody who lives here.Tu habites ici depuis longtemps. Tu dois tout le monde.

I don't seem to have my wallet. I must have it at home.Je n'ai pas mon portefeuille. Je dois l'avoir à la maison.

'How old is Ed?' 'He's older than me. He must at least 40.''Quel âge a Ed ?' 'Il est plus vieux que moi. Il doit au moins 40 ans.'

I didn't hear my phone. I must have asleep.Je n'ai pas entendu mon téléphone. Je devais dormir.

'You're going on holiday soon.' 'Yes, you must forward to it.''Tu pars bientôt en vacances.' 'Oui, tu dois avoir hâte.'

I'm sure you know this song. You must have it before.Je suis sûr(e) que tu connais cette chanson. Tu dois l'avoir avant.

The road is closed. There must an accident.La route est fermée. Il doit y eu un accident.

10.

'Do you have a car?' 'You must ! How could I afford one?''Tu as une voiture ?' 'Tu ! Comment pourrais-je en avoir une ?'

11.

David is the managing director of a large company. He must quite a high salary.David est directeur général d'une grande société. Il doit un salaire assez élevé.

Answers:Réponses : 1. dropped · 2. be · 3. know · 4. left · 5. be · 6. been asleep · 7. be looking · 8. heard · 9. have been / be · 10. be joking · 11. earn

EX 3

Exercise 3 — Write sentences with must have or can't have

Exercice 3 — Écrivez des phrases avec must have ou can't have

Use the words in brackets to write a full sentence.

Utilisez les mots entre parenthèses pour écrire une phrase complète.

We rang the doorbell but nobody answered. (they / go out)
→ (ils / sortir) →

Sarah hasn't contacted me. (she / get / my message)
→ (elle / recevoir / mon message) →

The jacket you bought is very good quality. (it / be / very expensive)
→ (elle / être / très chère) →

I haven't seen our neighbours for days. (they / go away)
→ (ils / être partis) →

I can't find my umbrella. (I / leave / it in the restaurant)
→ (je / laisser / au restaurant) →

Amy lost her job. It was a very difficult situation. (it / be / easy for her)
→ (ce / être / facile pour elle) →

A man was outside the café for a long time. (he / wait / for somebody)
→ (il / attendre / quelqu'un) →

Rachel did the opposite of what I asked. (she / understand / what I said)
→ (elle / comprendre / ce que j'ai dit) →

When I got back to my car, it was unlocked. (I / forget / to lock it)
→ (j' / oublier / de le verrouiller) →

10.

My neighbours were making a lot of noise. It woke me up. (they / have / a party)
→ (ils / faire / une fête) →

11.

The light was red, but the car didn't stop. (the driver / see / the red light)
→ (le conducteur / voir / le feu rouge) →

12.

Paul has had these shoes for years but they still look new. (he / wear / them much)
→ (il / les porter / beaucoup) →

Answers:Réponses : 1. They must have gone out · 2. She can't have got my message · 3. It must have been very expensive · 4. They must have gone away · 5. I must have left it in the restaurant · 6. It can't have been easy for her · 7. He must have been waiting for somebody · 8. She can't have understood what I said · 9. I must have forgotten to lock it · 10. They must have been having a party · 11. The driver can't have seen the red light · 12. He can't have worn them much

Frequently Asked Questions — Must & Can't

Questions fréquentes — Must & Can't

What is the difference between "must be" for obligation and "must be" for deduction?Quelle est la différence entre "must be" pour l'obligation et "must be" pour la déduction ?

Obligation: I must go now = I have to leave. Deduction: You must be tired = I'm certain you are tired based on evidence. Context always makes it clear.

Obligation : I must go now = je dois partir. Déduction : You must be tired = je suis certain(e) que tu es fatigué(e) d'après les preuves.

Can I use "mustn't" for deduction instead of "can't"?Peut-on utiliser "mustn't" pour la déduction à la place de "can't" ?

No. For negative deduction, use can't or couldn't, NOT mustn't. Mustn't = prohibition. ✓ That can't be right. ✗ That mustn't be right.

Non. Pour la déduction négative, utilisez can't ou couldn't, PAS mustn't. ✓ That can't be right. ✗ That mustn't be right.

What is the difference between "can't have" and "couldn't have" for past deduction?Quelle est la différence entre "can't have" et "couldn't have" pour la déduction passée ?

None — they mean exactly the same thing. She can't have got my message = She couldn't have got my message.

Aucune — ils ont exactement le même sens. She can't have got my message = She couldn't have got my message.

How do I know when to use "must be" vs "must have been"?Comment savoir quand utiliser "must be" vs "must have been" ?

Use must be for present deductions: You must be tired (now). Use must have been for past deductions: It must have been noisy (when you lived there).

Utilisez must be pour le présent, must have been pour le passé.

Can I use "must be" for continuous actions?Peut-on utiliser "must be" pour des actions continues ?

Yes. must be + -ing: She must be working late. For past: must have been + -ing: He must have been waiting for someone.

Oui. must be + -ing : She must be working late. Passé : must have been + -ing : He must have been waiting for someone.

⚙️ Quiz SettingsParamètres du Quiz

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All topics Must (Present) Can't (Present) Must Have (Past) Can't Have (Past) Mixed

Must & Can't Quiz

Quiz — Must & Can't

20 questions

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out ofsur 20 questionsquestions

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Correct

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Wrong

Incorrectes

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Skipped

Non répondues

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Must and Can't
Logical Deduction — Present & Past — Complete Guide Must et Can't
Déduction Logique — Présent & Passé — Guide Complet

Must & Can't — Logical Deductions About the Present

Must & Can't — Déductions logiques sur le présent

Must — Certain (logically true)

Must — Certain (logiquement vrai)

Can't — Impossible (logically false)

Can't — Impossible (logiquement faux)

Must Have & Can't Have — Deductions About the Past

Must Have & Can't Have — Déductions sur le passé

Must Have — Certain (past)

Must Have — Certain (passé)

Can't Have / Couldn't Have — Impossible (past)

Can't Have / Couldn't Have — Impossible (passé)

Must & Can't — Exercises with AnswersMust & Can't — Exercices avec Corrigé

Exercise 1 — Put in must or can't

Exercice 1 — Mettez must ou can't

Exercise 2 — Complete with a verb (one or two words)

Exercice 2 — Complétez avec un verbe (un ou deux mots)

Exercise 3 — Write sentences with must have or can't have

Exercice 3 — Écrivez des phrases avec must have ou can't have

Frequently Asked Questions — Must & Can't

Questions fréquentes — Must & Can't

Must & Can't — Quiz

Must & Can't — Quiz

⚙️ Quiz SettingsParamètres du Quiz

Must & Can't Quiz

Quiz — Must & Can't

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Evènements

Must and Can'tLogical Deduction — Present & Past — Complete Guide Must et Can'tDéduction Logique — Présent & Passé — Guide Complet

Must & Can't — Logical Deductions About the Present

Must & Can't — Déductions logiques sur le présent

Must — Certain (logically true)

Must — Certain (logiquement vrai)

Can't — Impossible (logically false)

Can't — Impossible (logiquement faux)

Must Have & Can't Have — Deductions About the Past

Must Have & Can't Have — Déductions sur le passé

Must Have — Certain (past)

Must Have — Certain (passé)

Can't Have / Couldn't Have — Impossible (past)

Can't Have / Couldn't Have — Impossible (passé)

Must & Can't — Exercises with AnswersMust & Can't — Exercices avec Corrigé

Exercise 1 — Put in must or can't

Exercice 1 — Mettez must ou can't

Exercise 2 — Complete with a verb (one or two words)

Exercice 2 — Complétez avec un verbe (un ou deux mots)

Exercise 3 — Write sentences with must have or can't have

Exercice 3 — Écrivez des phrases avec must have ou can't have

Frequently Asked Questions — Must & Can't

Questions fréquentes — Must & Can't

Must & Can't — Quiz

Must & Can't — Quiz

⚙️ Quiz SettingsParamètres du Quiz

Must & Can't Quiz

Quiz — Must & Can't

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Must and Can't
Logical Deduction — Present & Past — Complete Guide Must et Can't
Déduction Logique — Présent & Passé — Guide Complet