Must / Mustn't

1. Overview & Concept

• What is it? Must and mustn't (must not) are modal verbs that express strong obligation, necessity, and prohibition.
• Purpose: We use must to say that something is necessary or strongly required — either by an external rule or by the speaker's own judgment. We use mustn't to say that something is forbidden or strongly not recommended.

2. Structure & Formula

Affirmative:

Subject + must + base verb

Negative:

Subject + mustn't / must not + base verb

Question (rare in everyday use):

Must + subject + base verb?

• Must you leave so soon? (formal/literary)
• In practice, questions about necessity more often use Do you have to…?

Key grammar rules:
- Must has no past tense form of its own. Use had to for past obligation.
- Must never changes form — no -s, -ed, or -ing.
- Always followed by a bare infinitive.

3. Usage Rules

• Rule 1 — Strong obligation (internal): Use must when the obligation comes from the speaker's own opinion or strong conviction.
• I must call my mother — it's her birthday today.

• You must see this film — it's incredible.

• Rule 2 — Rules and regulations: Use must for official rules, signs, and instructions.

• Passengers must wear seatbelts at all times.

• You must show your ID at the entrance.

• Rule 3 — Prohibition (mustn't): Use mustn't to say something is not allowed or strongly forbidden.

• You mustn't talk during the exam.
• Children mustn't play near the road.

Critical distinction — mustn't vs. don't have to:

• Rule 4 — Logical deduction (strong certainty — B2+): Must is also used to deduce something is certainly true. (See topic: Must/Can't for Logical Deduction)

4. Signal Words & Context Clues

• Obligation context: phrases like it is essential that, it is necessary, regulations state, the rules say.
• Prohibition context: No…, Do not…, Forbidden to… — these notices often use must not in formal writing.
• Personal conviction: I really think…, It's so important that…

5. Common Pitfalls & Mistakes

• ❌ She musts go. → ✅ She must go. (No -s on modal verbs.)
• ❌ He must to finish by Friday. → ✅ He must finish by Friday. (No to after modal.)
• ❌ You mustn't to smoke here. → ✅ You mustn't smoke here.
• ❌ I musted call him yesterday. → ✅ I had to call him yesterday. (Must has no past tense; use had to.)
• ❌ You don't must do that. → ✅ You mustn't do that. (Negative of must is mustn't, not don't must.)
• ❌ Confusing mustn't and don't have to: You mustn't come (forbidden) ≠ You don't have to come (optional).

6. Real-World Examples

1. You must wear a helmet when riding a motorcycle.
2. I must finish this report before the meeting.
3. Students mustn't use their phones during class.
4. You must show your boarding pass at the gate.
5. We must leave now or we'll miss the train.
6. Employees mustn't share confidential information.
7. You must try this restaurant — the food is amazing.
8. Children under 15 mustn't watch this film.
9. All visitors must sign in at the reception desk.
10. I must buy some milk on the way home.

7. Summary Table