Module 4: Mastering Modal Verbs - Lesson 4.8: Modal Verb of Prohibition: MUST NOT (MUSTN'T)

Lesson 4.8: Modal Verb of Prohibition - MUST NOT (MUSTN'T)

In this lesson, we will focus on the modal verb phrase MUST NOT (and its contraction MUSTN'T), which is specifically used to express prohibition in English. Prohibition means that something is not allowed or is forbidden. MUST NOT is a strong and direct way to indicate that an action is not permitted and should not be done.

MUST NOT / MUSTN'T - Expressing Prohibition

MUST NOT (or MUSTN'T) is used exclusively to express prohibition. It indicates a strong and clear ban or interdiction against doing something. It signifies that an action is against rules, laws, or strong orders and is not permissible under any circumstances.

Key Uses of MUST NOT / MUSTN'T (Prohibition):

Expressing Strong Prohibition: To clearly and forcefully state that something is not allowed, is forbidden, or is against the rules.
Rules and Regulations (Strong Form): Often used for rules, laws, and regulations where there is a strict and non-negotiable prohibition.
Warnings against Undesirable Actions: Can be used to strongly warn against doing something that is dangerous, harmful, or strongly discouraged.

Forming Sentences with MUST NOT / MUSTN'T (Prohibition):

The structure is:

Formula: Subject + must not / mustn't + Base Verb

Examples of MUST NOT / MUSTN'T (Prohibition) in Use:

Expressing Strong Prohibition (Rules and Regulations):
- "You must not park here. This is a private zone." (Strict rule - parking is forbidden)
- "Students must not use mobile phones in the exam room." (Exam regulation - phone use is strictly prohibited)
- "Visitors must not touch the exhibits in the museum." (Museum rule - touching is forbidden)
- "You must not smoke in this building." (Building rule/law - smoking is prohibited indoors)
- "Employees must not disclose confidential information." (Company policy - disclosure is forbidden)
Warnings against Undesirable Actions:
- "You mustn't forget your passport when you travel abroad." (Strong warning against forgetting - essential item)
- "You mustn't drink and drive. It's dangerous and illegal." (Strong warning against dangerous and illegal action)
- "Children mustn't play near the railway tracks." (Strong warning against a very dangerous activity)
- "You mustn't rely on rumors; check the facts." (Strong advice/warning against an unreliable source of information)

Negative and Question Forms of MUST NOT / MUSTN'T (Prohibition):

The form "must not" is already negative and expresses prohibition. Therefore:

Negative: "Must not" itself *is* the negative form and expresses prohibition. There isn't a further negative form of "must not" for prohibition. To express that something *is* allowed, you would use modals of permission like "can" or "may" (as covered in Lesson 4.5).
Question: Asking questions with "must not" to express prohibition is grammatically possible but very uncommon and often awkward. To ask about rules or prohibitions, it's more natural to rephrase using other structures (e.g., "Is it allowed to...?", "Are we permitted to...?"). Questions with "must not" are very rare and would typically be rhetorical or express strong surprise/disbelief at a prohibition.

Less Common/Awkward Question Examples (for informational purposes - avoid using these in normal conversation for prohibition):

"Mustn't we park here?" (Very awkward and unusual way to ask if parking is prohibited - more likely to be rhetorical, expressing surprise that parking might be forbidden). Instead, use: "Is it forbidden to park here?", "Are we allowed to park here?", "Can we park here?"
"Mustn't students use mobile phones in the exam?" (Very unusual way to ask about a rule. More natural to ask: "Are students allowed to use mobile phones in the exam?", "Is it forbidden for students to use mobile phones...?")

Instead of questioning with "must not" for prohibition, rephrase using:

"Is it allowed to...?" / "Are we allowed to...?" (For permission - opposite of prohibition)
"Is it permitted to...?" / "Are we permitted to...?" (Formal permission)
"Is it forbidden to...?" / "Are we forbidden to...?" (Directly asking about prohibition)
"Can we...?" / "Could we...?" (Informal/Polite requests for permission - implying checking if something is *not* prohibited)

MUST NOT / MUSTN'T vs. CANNOT / CAN'T for Prohibition: Key Differences - Strength and Formality

MUST NOT / MUSTN'T: Expresses strong prohibition. Clear and direct ban. Often used for rules, laws, strong warnings. Can sound more formal and authoritative than "cannot" for prohibition.
CANNOT / CAN'T: Can also express prohibition, but often is more general and can also indicate inability. For prohibition, "cannot" can sometimes sound slightly less forceful than "must not," and can also be used for expressing lack of permission in informal contexts (as we saw in Lesson 4.5).

While both can convey prohibition, MUST NOT/MUSTN'T is generally the stronger and more emphatic choice for expressing that something is strictly forbidden, especially in rules, regulations, and strong warnings. "Cannot/can't" can also express prohibition, but may sometimes be interpreted as also relating to inability, and is often slightly less formal in tone for prohibition. When you want to be absolutely clear and forceful about something being forbidden, MUST NOT/MUSTN'T is the most direct and impactful modal verb phrase.

Key takeaway: MUST NOT (MUSTN'T) is used to express strong prohibition - that something is strictly forbidden or not allowed. It is used for rules, laws, and strong warnings. It is a forceful way to indicate that an action is not permitted. To ask about prohibitions, rephrase questions using "Is it allowed...?", "Is it forbidden...?", or "Can we...?". Next, we will explore Modal Verb of Lack of Necessity/Obligation: DON'T HAVE TO / NEEDN'T!