English Modal Verbs of Prohibition: Avoid Pitfalls in the TOEIC®
Flow Exam team
Prohibition modals are used to express what is forbidden or discouraged in English.
In the TOEIC®, they appear most often in Part 5 and 6, and also in Part 7 within company regulations or policies.
The trap that comes up constantly: confusing mustn't (strict prohibition) with don't have to (no obligation). These are two structures that test-takers regularly mix up under pressure.
The Three Levels of Prohibition to Know
Strict Prohibition: mustn't and may not
Mustn't expresses a formal prohibition, often related to a safety rule or internal regulation.
- "Employees mustn't park in the visitor area."
May not (more formal) appears in official documents, contracts, or policies.
- "Contractors may not access the premises after 7 PM."
Legal Prohibition: cannot / can't
Cannot (contracted form: can't) expresses a legal or regulatory prohibition, or an absolute impossibility.
- "Participants cannot register after the deadline."
Strong Negative Advice: shouldn't / had better not
Shouldn't expresses negative advice, without formal prohibition.
- "You shouldn't send personal emails during work hours."
Had better not (more direct) indicates a warning with a potential consequence.
- "We had better not miss the meeting with the investors."
TOEIC® Trap #1: mustn't vs don't have to
Based on our experience with the candidates we coach, this confusion costs the majority of students points, even those who are strong in grammar.
| Structure | Meaning | TOEIC® Example |
|---|---|---|
| mustn't | strict prohibition | "You mustn't disclose confidential information." |
| don't have to | absence of obligation (= it's optional) | "You don't have to attend the training session." |
The life-saving reflex: if you can mentally replace it with "it is forbidden to", use mustn't. If you can say "it is not necessary to", use don't have to.
Classic Part 5 error:
- "Visitors don't have to enter the restricted zone." -> Incorrect
"Visitors mustn't enter the restricted zone." -> Correct
The first one means it's optional; the second means it's forbidden. In the TOEIC®, the context (restricted zone) clearly indicates a prohibition.
Recognizing Context Clues in Part 5 and 6
The TOEIC® never tests grammar in a vacuum. Every question gives you clues to choose the correct modal.
Strict Prohibition Clues
Keywords that call for mustn't or may not:
- prohibited, restricted, forbidden
- safety regulations, company policy
- confidential, unauthorized
"Access to the server room is restricted. Staff members _____ enter without authorization." → mustn't (safety regulation)
Negative Advice Clues
Keywords that call for shouldn't:
- advisable, recommended, suggest
- it is better to, preferable
"It is not advisable to submit incomplete reports. You _____ send them before reviewing all data." → shouldn't (advice)
Summary Table of Frequent Errors
| Error | Correction | Why it's a TOEIC® Trap |
|---|---|---|
| You mustn't to leave early. | You mustn't leave early. | Modals never take "to" |
| He don't have to smoke here. | He mustn't smoke here. | Confusing prohibition / absence of obligation |
| Employees may not to use personal devices. | Employees may not use personal devices. | Extra "to" after modal |
| You better not be late. | You had better not be late. | Incomplete form of the modal |
Practical Method for Part 5
Even among candidates who already use a preparation platform through their school, hesitation between prohibition modals comes up often. These tools emphasize the rule, but rarely the reflex to adopt under pressure.
3-Step Decision Checklist
Step 1: Read the entire context (not just the fill-in-the-blank sentence)
Step 2: Identify the type of constraint
- Regulation / safety / legal → strict prohibition
- Advice / recommendation → negative advice
- No obligation → don't have to (BUT watch out for the trap)
Step 3: Check that there is no "to" after the modal
Candidates who progress quickly all do the same thing: they read the ENTIRE sentence before choosing, not just the words around the blank.
Typical TOEIC® Practice Cases
Example 1: Internal Email
"Dear team, please note that employees _____ use the company parking on weekends without prior approval."
A) don't have to B) mustn't C) shouldn't to D) may not to
Answer: B
Explanation -> "Prior approval" indicates a strict rule. C and D contain "to" (impossible after a modal). A would change the meaning (= optional).
Example 2: Company policy
"According to the new policy, staff members _____ share login credentials with external partners."
A) don't must B) mustn't C) haven't to D) not should
Answer: B
Explanation -> "Policy" + "login credentials" = strict security prohibition. The other options are grammatically incorrect.
Ready to train?
Prohibition modals represent a common grammar point on the TOEIC®, especially in Part 5 where formulation traps are numerous. The reflex between mustn't and don't have to often makes the difference between 700 and 850 points.
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