What are "Must" and "Mustn't" in English Grammar?
Must and its negative form mustn't (must not) are strong modal verbs used to express obligations, absolute necessities, and strict prohibitions.
When you use must, you are stating that an action is vitally important or required. When you use mustn't, you are stating that an action is strictly forbidden or highly dangerous.
Like all true modal verbs, must does not change form (there is no "-s" in the third person), and it is always followed by a bare infinitive (a verb without "to"). Furthermore, must only exists in the present tense; it has no past or future forms.
"Must" and "Mustn't" Structure and Formula
The sentence structure is simple because the modal never changes depending on the subject.
Standard Formula:
- Positive: Subject + must + base verb + Object
- Negative: Subject + mustn't / must not + base verb + Object
Pedagogical shorthand:
- (+) S + must + V(bare) + O
- (-) S + mustn't / must not + V(bare) + O
How to Form "Must" and "Mustn't"
Positive (Affirmative) Sentences
Use must to declare a strong obligation or necessity.
| Subject (S) | Modal | Main Verb (V_bare) | Object/Complement |
|---|---|---|---|
| I / We / They | must | wear | a seatbelt while driving. |
| He / She / It | must | finish | the project today. |
Example: You must submit your application before the deadline.
Negative Sentences (Prohibition)
Use mustn't to forbid an action entirely. Note that must not is often used in formal writing (like signs), while mustn't is more common in speech.
| Subject (S) | Modal | Main Verb (V_bare) | Object/Complement |
|---|---|---|---|
| You / They | mustn't | touch | the hot stove. |
| He / She | must not | park | in the disabled space. |
Example: Passengers must not unfasten their seatbelts until the plane stops.
Questions
Questions starting with "Must" (e.g., Must you leave so soon?) exist but are considered highly formal, dramatic, or old-fashioned. In modern English, we almost always use Do/Does... have to? to ask about obligations.
- Formal/Rare: Must we pay now?
- Common/Natural: Do we have to pay now?
When to Use "Must" and "Mustn't" in English
Understanding the specific context for must is critical, especially so it is not confused with have to.
1. Internal / Personal Obligation
In spoken English, must is most often used when the speaker personally feels something is very important. The obligation comes from the inside, not from an external law.
- I must call my mother today; it's her birthday! (I feel this is important).
- You simply must see that new movie; it is fantastic! (A strong personal recommendation).
2. Formal Rules and Written Regulations
In formal writing, signs, and official documents, must is the standard word used to declare rules and laws.
- All visitors must sign in at the front desk.
- Employees must wash their hands before returning to work.
3. Strict Prohibition
Mustn't is universally used to forbid something. It means "do not do this under any circumstances."
- You mustn't tell anyone my secret.
- Children mustn't play near the busy road.
4. Logical Deduction (Certainty)
Must is also used to deduce that something is logically true based on evidence (e.g., You've been driving for 10 hours; you must be tired). For a deeper dive into this, see the dedicated topic: Must/Can't for Logical Deduction.
Common Signal Words for "Must" and "Mustn't"
- For obligation: it is essential that..., the rules state..., absolutely necessary
- For prohibition: forbidden, not allowed, illegal, do not...
- Personal urgency: I really think..., I promise I will...
How to Tell the Difference Between "Must" and Similar Grammar Topics
"Must" vs. "Have to"
- Must is for internal, personal obligations, or formal written rules. (I must exercise more.)
- Have to is for external, circumstantial obligations imposed by an boss, authority, or situation. (I have to work on Saturday.)
- Important Note: In everyday spoken English, have to is far more common than must for general obligations.
"Mustn't" vs. "Don't Have To"
This is the most critical distinction in English modals!
- Mustn't means the action is forbidden/prohibited. (Do NOT do it).
- You mustn't drive through a red light.
- Don't have to means the action is not necessary / optional. (You can do it if you want, but you don't need to).
- You don't have to bring a gift to the party.
Past Tense of "Must"
Must does not have a past tense form! If you want to talk about a strong obligation that occurred in the past, you must switch to the past form of have to: had to.
- Present: I must study tonight.
- Past: I had to study last night. (NOT "I musted" or "I must have").
Common Mistakes with "Must" and "Mustn't"
- ❌ Mistake: Adding an "-s" to the modal.
- Incorrect: He musts go right now.
- Correct: He must go right now.
- ❌ Mistake: Using "to" after the modal.
- Incorrect: You must to listen to me!
- Correct: You must listen to me!
- ❌ Mistake: Trying to use must in the past tense.
- Incorrect: Yesterday I must pay the bill.
- Correct: Yesterday I had to pay the bill.
- ❌ Mistake: Confusing the negative meaning.
- Incorrect: It's the weekend! You mustn't work today! (Unless working is literally forbidden).
- Correct: It's the weekend! You don't have to work today!
Real-life Examples of "Must" and "Mustn't" Usage
- (Internal Obligation): I have a terrible toothache; I really must go to the dentist.
- (Strong Recommendation): You must try the chocolate cake here—it's phenomenal.
- (Written Rule): All passengers must fasten their seatbelts during turbulence.
- (Prohibition): Students mustn't use their mobile phones under any circumstances while in the exam hall.
- (Internal Obligation): We must buy John a present before his party tonight.
- (Written Rule): Hard hats and safety boots must be worn on this construction site.
- (Prohibition): You mustn't walk on the grass; the sign clearly forbids it.
- (Prohibition): I mustn't forget to lock the back door before going to bed.
- (Past Obligation — had to): The car broke down, so we had to walk five miles in the rain.
- (Formal Question): (Rare) Must we endure this terrible music the entire journey?
Summary & Cheatsheet for "Must" and "Mustn't"
| Meaning/Context | Modal | Formula Structure | Example |
|---|---|---|---|
| Internal Obligation | must | S + must + V(bare) | I must remember her name. |
| Formal Rule | must | S + must + V(bare) | You must sign the register. |
| Strong Recommendation | must | S + must + V(bare) | You must visit Paris! |
| Strict Prohibition | mustn't | S + mustn't + V(bare) | You mustn't smoke here. |
| Past Obligation | had to | S + had to + V(bare) | I had to leave early. |
Frequently Asked Questions
Is it wrong to use "have to" instead of "must"?
Generally, no. In affirmative sentences, native speakers often use "have to" and "must" interchangeably in everyday speech. However, if you are emphasizing that the obligation is your idea, "must" is better. Remember, in the negative, they have completely different meanings ("mustn't" vs "don't have to").
Why don't we use "must" for questions?
In old English, asking "Must I go?" was common. Today, it sounds incredibly theatrical, dramatic, or aristocratic. It is much more natural and common to use "Do I have to go?".
How do I pronounce "mustn't"?
The first "t" is silent. It is pronounced /mʌsnt/ (MUSS-ent), not /mʌstnt/.