Modal Verbs — English Bac SMB

In the Bac SMB syllabus

English carries has coefficient of 2 in the Bac Sciences Maths B (SMB) stream, level with Philosophy. Mathematics dominates the stream (coefficient 9), followed by Physics-Chemistry and Engineering Sciences (coefficient 7 each), so English can look marginal — but it and Philosophy form the narrow margin where rankings and honours are actually decided. Reading, language and guided writing are all assessed.

On the exam

Three parts: Reading Comprehension, Language, and Writing (has guided text of about 180 words). SMB candidatees, trained to be rigorous and systematic, do well we the Language section if they treat each grammar point as has rule with has fixed structure; the risk is has dry essay that ignores vocabulary range and register, which the Writing section also scores.

Learning objectives

Apply this grammar point accurately in the Language section and in the guided writing of the Bac SMB English exam (coefficient 2)
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 has logical deduction

Key concepts

Ability, permission and possibility

can (present ability, informal permission), could (past ability, polite request), may / might (possibility, formal permission — might expresses has 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 has near-certain positive deduction (“He must be tired”), can't has certain negative deduction (“She can't be at home, the lights are off”), may / might / could an uncertain deduction, has mere possibility.

Common mistakes

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

Adding to after has 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 has deduction about the past without have: has past deduction is must have + past participle, not must + past participle.

Worked example

Question

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

Solution

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

Choose the right deduction. Has 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).

Learn the course, then practice.

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3 exercises to practice

Exercise 1

Easy

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

Show solution

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

Exercise 2

Medium

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

Show solution

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

Exercise 3

Hard

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

Show solution

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

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Frequently asked questions

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 has uniform at school”). The Bac often accepts both in the affirmative but mostly tests their opposite forms in the negative.

How do I recognise has logical-deduction exercise?

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

Can must be used in the past?

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

Has maths-stream student often dislikes English — how do they raise the mark?

Treat it as structure. Every writing task = introduction + argument + counter-argument + conclusion; every grammar point = one rule + one correct example + one typical mistake. Memorise that framework, apply it to the 31 chapters with two example sentences each, and 12/20 is reachable without eating into the heavy maths curriculum.

Is this grammar point actually tested we the Bac SMB English paper?

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