1. A prohibition requires not doing the action
In its prohibition meaning, mustn’t means an action is not allowed or must be avoided. The speaker does not leave the action as a choice. Understand the rule as a need to refrain from an action, rather than an absence of a need to perform it.
All rules for places and events in this lesson are invented for English practice. They do not state real legal, medical or safety requirements.
You mustn't open this box.
You mustn't open this box.
The box’s owner prohibits opening it, rather than simply saying it is optional.
Members must not take these books home.
Members must not take these books home.
The statement sets a limit on what all members are allowed to do.
2. One form for every subject
After the subject, put mustn’t or must not, followed by the base verb without to: use, touch, bring. With he, she and it, add -s to neither must nor the next verb.
If the meaning needs be, use be after mustn’t, rather than am/is/are. Do not add don’t or doesn’t for the negative: not already belongs to must.
Subject + must not / mustn’t + base verb
- subject / S —
- The grammatical subject: who or what the clause is about; the verb agrees with it. In The book was written, the book is the subject, although someone else does the action.
- verb / V —
- A word for an action or state, such as work or be. The pattern determines its form.
- V1 / base form —
- The base verb without to: work, go, be.
She mustn't move the chairs.
She mustn't move the chairs.
The subject she does not change move after the modal.
We mustn't be noisy here.
We mustn't be noisy here.
Be connects the modal with the adjective noisy.
3. Full and contracted forms
Must not and mustn’t express the same basic requirement in a prohibition. The contraction is natural in conversation and informal writing; the full form often appears in notices or is used for emphasis.
Write mustn’t with an apostrophe before t; full must not is two words. A full form is not incorrect in conversation, and a contraction does not itself turn a prohibition into advice.
You mustn't draw on the table.
You mustn't draw on the table.
A conversational formulation of a rule.
You must not draw on the table.
You must not draw on the table.
The same prohibition in full form; it can sound emphatic.
4. Prohibited, optional or better avoided?
Mustn’t prohibits. Don’t have to and don’t need to usually mean an action is unnecessary or optional: there is no obligation. Do not automatically equate negating necessity with prohibiting an action; first identify the intention.
Shouldn’t often gives negative advice about what is better avoided. It is grammatical but not equivalent to a clear rule with mustn’t. Can’t and not be allowed to can also express prohibition. If an exercise requests mustn’t / must not, another construction may be correct English without meeting that learning instruction.
You mustn't bring food into this room.
You mustn't bring food into this room.
Under the fictional rule, food is prohibited.
You don't have to bring food to the meeting.
You don't have to bring food to the meeting.
This is not a prohibition: we only state there is no need.
You shouldn't bring a big bag on this short walk.
You shouldn't bring a big bag on this short walk.
Advice for convenience, rather than an announced prohibition.
5. A rule for others, or a reminder to yourself
Mustn’t is not limited to addressing you. We includes the speaker in the group; I can express a firm requirement for yourself. A group name such as guests specifies who the rule applies to.
A negative verb does not necessarily mean a negative result: I mustn’t forget means remembering is necessary. You can name a future situation with tonight or tomorrow without adding will before mustn’t.
We mustn't tell Nina about the surprise.
We mustn't tell Nina about the surprise.
The speaker also commits to keeping the secret.
I mustn't forget my ticket tomorrow.
I mustn't forget my ticket tomorrow.
The reminder means remembering to take the ticket, rather than leaving it at home.
Guests mustn't use this door tonight.
Guests mustn't use this door tonight.
Tonight gives future reference; mustn’t stays unchanged.
6. Context matters more than automatic translation
Not every must not / mustn’t in English is a literal rule. In a friendly remark such as You mustn’t worry, the speaker may be reassuring someone rather than prohibiting a feeling. Strength and function depend on the situation.
Must not / mustn’t can also occur in deductions from evidence, meaning something is probably not the case. We do not practise these deductions in this lesson. In the core prohibition meaning, it is always clear which action the rule or speaker does not allow.
Don’t have to sometimes functions as an indirect reproach, such as when someone is shouting. “No obligation” is therefore its basic meaning, rather than a mechanical translation of every conversational remark.
You mustn't worry about the party. Everything is ready.
You mustn't worry about the party. Everything is ready.
This is support, rather than a rule with a penalty for breaking it.
He must not know our new address; he sent the card to the old one.
He must not know our new address; he sent the card to the old one.
The speaker makes a deduction, rather than prohibiting knowledge of the address. This is a boundary example, rather than a productive A2 target.
You don't have to shout. I'm right here.
You don't have to shout. I'm right here.
In this conversation, it is an indirect request to stop shouting, rather than a statement about an optional task.
7. Formulating an appropriate prohibition
First identify who the rule addresses, exactly which action is prohibited, and where or when the restriction applies. Then build a short sentence. A reason or permitted alternative helps the listener decide what to do next.
Do not turn a rule for one place into a general prohibition. For example, prohibiting audio recording in a room does not prohibit all phones in the building. Open answers allow appropriate synonyms and your own phrasing; the model is a guide, rather than the only correct expression.
You mustn't record the talk, but you can take notes.
You mustn't record the talk, but you can take notes.
The prohibition concerns recording, rather than listening or taking notes.
We mustn't leave cups on this desk because it is for drawings.
We mustn't leave cups on this desk because it is for drawings.
We covers everyone in the group; the reason explains the specific rule.
Common mistakes
Incorrect:You mustn't to touch the model.
Correct:You mustn't touch the model.
After mustn’t, use touch without to. Remove only the unnecessary to.
Incorrect:She mustn't takes photos here.
Correct:She mustn't take photos here.
After the modal, take does not receive -s, even with she.
Incorrect:They don't must enter this room.
Correct:They must not enter this room.
Form the negative with not after must, without auxiliary don’t.
Incorrect:We mustn't are late.
Correct:We mustn't be late.
After mustn’t, use base be, rather than finite are.