Modal Verbs — English Bac Lettres

Dans le programme du Bac Lettres

English is a major subject in the Bac Lettres stream, with a coefficient of 4 — second only to Arabic Language (coefficient 7) and ahead of History-Geography and Philosophy (coefficient 3). There is no Mathematics in the stream. The high coefficient raises the bar: richer vocabulary, more developed argumentative writing, and firmer control of register and style are expected, and the English mark genuinely shapes the average.

À l'épreuve

Three parts: Reading Comprehension, Language, and Writing. In the Lettres stream the Writing task is longer and more demanding — a structured essay with a clear thesis, developed arguments on both sides, illustrative examples and a personal conclusion — and language accuracy and range are scored throughout.

Objectifs d'apprentissage

Apply this grammar point accurately in the Language section and in the guided writing of the Bac Lettres English exam (coefficient 4)
Distinguish can / could (ability, informal permission) from may / might (possibility, formal permission)
Distinguish must (obligation imposed by the speaker) from have to (obligation imposed by an outside rule)
Contrast must not (prohibition) with don't have to (absence of obligation)
Use must, can't and may / might / could to express a logical deduction

Notions clés

Ability, permission and possibility

can (present ability, informal permission), could (past ability, polite request), may / might (possibility, formal permission — might expresses a weaker possibility than may).

Obligation and advice

must (obligation the speaker imposes), have to (obligation an outside rule imposes); must not (prohibition) and don't have to (absence of obligation, the action is still allowed) are opposite meanings; should / ought to (advice, recommendation).

Logical deduction

must expresses a near-certain positive deduction (“He must be tired”), can't a certain negative deduction (“She can't be at home, the lights are off”), may / might / could an uncertain deduction, a mere possibility.

Erreurs fréquentes

Confusing must not (prohibition) with don't have to (absence of obligation, the action is still allowed) — opposite meanings.

Adding to after a modal: “must to go” is wrong — must is always followed by the base form without to (have to is the exception, since have is the finite verb).

Using must for a deduction about the past without have: a past deduction is must have + past participle, not must + past participle.

Exemple corrigé

Énoncé

Choose the right modal and justify: “The lights are off and the car isn't in the driveway. They ___ be at home.” (must / can't / should)

Correction

Weigh the evidence. Lights off and car gone are strong evidence that they are NOT at home.

Choose the right deduction. A certain negative deduction is marked by can't, not must (which would mean the opposite).

Result. “They can't be at home.” (must would mean “they must be at home”, which contradicts the evidence).

Le cours, puis l'entraînement.

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3 exercices pour t'entraîner

Exercice 1

Easy

Complete with must or mustn't: “You ___ smoke in the hospital, it's strictly forbidden.”

Voir la correction

mustn't — a formal prohibition, not an absence of obligation.

Exercice 2

Medium

Complete with have to or don't have to: “It's Saturday, so I ___ go to work today.”

Voir la correction

don't have to — absence of obligation (it is Saturday, going to work is not required), not a prohibition.

Exercice 3

Hard

Correct: “She must has finished the report by now.”

Voir la correction

A deduction about a completed fact is formed with must have + past participle: “She must have finished the report by now.”

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Questions fréquentes

must and have to — what is the real difference in the exam?

In meaning both express obligation, but must comes from the speaker (“I must call my mother”) and have to comes from an outside rule or authority (“I have to wear a uniform at school”). The Bac often accepts both in the affirmative but mostly tests their opposite forms in the negative.

How do I recognise a logical-deduction exercise?

The context gives clues (“the lights are off”, “she got an A”) from which you must conclude: must for a near-certain positive conclusion, can't for a certain negative conclusion, might / may / could for a mere possibility.

Can must be used in the past?

must itself has no past form for obligation (use had to); but for a deduction about a past fact, keep must and add have + past participle: must have finished, must have left.

English is coefficient 4 in the Lettres stream — how do I revise it seriously?

Treat it as a core subject. For each of the 31 chapters, fix the rule or the word field with two solid examples, and write at least one full essay a fortnight under timed conditions. Read short English articles to build argumentative vocabulary and sentence variety, since range and accuracy are marked directly.

Is this grammar point actually tested on the Bac Lettres English paper?

Yes — in the Language section as gap-fill, sentence-joining or error-correction, and indirectly in Writing, where the same structures decide whether a sentence is marked as accurate. One rule, two examples and one typical mistake per point is enough to cover both.