Expressing prohibition in English: mustn’t, can’t, forbidden to…

English has several ways to express prohibition, each with its own degree of firmness and its own register. This sheet details the modals and structures to use depending on the context, the classic trap between prohibition and absence of obligation, as well as the formulations found on signs.

Mustn’t: strict and formal prohibition

Mustn’t (or must not) is the negative form of must. Where must expresses obligation, its negative form expresses its exact opposite: prohibition, with all the firmness of the original modal.

• Employees must not share their passwords. → Employees must not share their passwords.
• You mustn’t leave personal documents on your desk overnight. → You must not leave personal documents on your desk overnight.
• Visitors must not enter this area without a badge. → Visitors must not enter this area without a badge.

Like other pure modals, mustn’t is only conjugated in the present tense.

Can’t / cannot: common prohibition

Can’t also expresses a prohibition, but in a more everyday and less formal register than mustn’t. It is commonly used orally to say that an action is not allowed or is not possible in the given context.

• You can’t use your phone during the meeting. → You cannot use your phone during the meeting.
• Employees cannot wear jeans on weekdays. → Employees cannot wear jeans on weekdays.
• You can’t park here. → You cannot park here.

In the past tense, can’t becomes couldn’t; for other tenses, not able to is used.

Can’t is also used to express a logical or emotional impossibility rather than a formal rule: You can’t be serious! does not refer to any regulatory prohibition, but to the speaker’s disbelief.

May not: polite and official prohibition

May not expresses a prohibition or refusal of permission in a polite and formal way, often in regulations or directives. It is equivalent to “not being allowed to”.

• Employees may not leave the office before 5 p.m. without prior approval. → Employees are not allowed to leave the office before 5 p.m. without prior approval.
• Visitors may not take pictures in this facility. → Visitors are not allowed to take pictures in this facility.

Not allowed to / not permitted to: prohibition in all tenses

Unlike pure modals, these two phrases can be conjugated in any tense, making them the default choice outside the present tense.

Forbidden to / prohibited from: legal and regulatory prohibition

These two formulations, also compatible with all tenses, are reserved for a very formal register: regulations, legal texts, institutional contexts. They emphasize a firm prohibition, sometimes accompanied by a sanction.

• Employees are forbidden to use the company car for personal trips. → Employees are forbidden to use the company car for personal trips.
• You are prohibited from drinking alcohol on these premises. → You are prohibited from drinking alcohol on these premises.

Note the difference in construction: forbidden to + infinitive, but prohibited from + verb in -ing.

Should not: the warning close to prohibition

Shouldn’t is not strictly speaking a modal of prohibition: it expresses a strong piece of advice not to do something, sometimes perceived as an indirect prohibition. It is often found in safety instructions or recommendations.

• You shouldn’t leave your workstation unlocked. → You should not leave your workstation unlocked.
• You shouldn’t swear in front of your grandma! → You should not swear in front of your grandma!

The trap: mustn’t vs don’t have to

Because have to expresses obligation, one might think that its negative form don’t have to expresses prohibition, like mustn’t. This is a common mistake: don’t have to actually expresses the absence of obligation, not prohibition.

In the first case, answering is prohibited. In the second, answering remains possible, just not necessary.

The verb to ban

The verb to ban expresses a strong prohibition, often total and permanent, decided by an authority (government, institution, company).

• The government banned smoking in public places. → The government banned smoking in public places.
• The airline banned electronic devices on certain flights. → The airline banned electronic devices on certain flights.

Prohibitions on signs

English signage uses short, fixed formulations, often built with No + verb in -ing or No + noun, rather than a full modal.

Which modal to choose depending on the context

The choice depends not only on the degree of prohibition, but also on the expected register.

• Formal or professional context (regulations, safety instructions, official documents): must, mustn’t, may not. Visitors mustn’t enter the laboratory without permission.
• Everyday life, informal conversation: can’t, don’t have to, shouldn’t. You can’t eat in the living room.
• Nuanced recommendation, without authority: should, shouldn’t, be supposed to. You shouldn’t leave the door open.

Using may not in a too familiar conversation sounds cold and distant: You may not borrow my pen seems out of place between friends, whereas You can’t borrow my pen right now remains natural. Conversely, reserving can’t for an official regulation weakens the seriousness of the message.

Summary: strength and register of each formulation

Practical exercise

Check your knowledge on expressing prohibition: