🧭 Grammar · Modal Verbs
Modals of Obligation & Permission (Must, Have To, Can)
Must, have to, mustn’t, don’t have to, can, could — the modals that tell you what’s required, what’s forbidden, and what’s allowed.
Grammar › Modal Verbs › Modals of Obligation & Permission (Must, Have To, Can)
The rule in one sentenceUse must or have to for obligation, mustn’t for what’s forbidden, don’t have to for what’s optional, and can or could for permission — four very different meanings hiding behind similar-looking words.
What are modals of obligation and permission?
These are the small verbs English uses to talk about rules, duty and what’s allowed. Must expresses strong obligation, often coming from the speaker’s own judgement: Andy Sachs thinks, “I must call Miranda back before she lands,” in The Devil Wears Prada. Have to expresses obligation that comes from outside — a law, a boss, a system: Actors have to join the union before working on a Marvel film. Can and could handle permission: Can I bring a guest to the Devil Wears Prada 2 premiere?
The two negatives are where learners trip up, because they look like a matching pair but mean opposite things. Mustn’t means something is forbidden — don’t do it. Don’t have to means something is simply not necessary — you can do it if you want, but nobody is making you. In The Devil Wears Prada, Miranda Priestly’s assistants mustn’t be late; but on her one day off, they don’t have to answer their phones.
🎬 Compare: “In The Devil Wears Prada, interns at Runway mustn’t take unpaid days off” (forbidden — a rule) vs “Interns don’t have to wear designer labels, but most do anyway” (optional — no rule against ordinary clothes).
Same grammatical shape, opposite meaning. This is the single most tested contrast in this topic.
Formal permission has its own polite cousin, may: Asking to speak with Miranda privately, the way Andy does in The Devil Wears Prada, is more formal than “Can I speak with you privately?” — both are correct, but may suits a job interview or a courtroom drama like A Few Good Men more than a text message to a friend.
How to build each modal
Must — strong obligation / rules
Every intern at Runway in The Devil Wears Prada must arrive before 7 a.m.
Staff at Runway mustn’t discuss Miranda Priestly’s decisions with the press.
mustn’t = forbidden, not just “not necessary”Must Andy wear designer labels to the Devil Wears Prada 2 after-party?
Must + subject + base verb — no “do”Have To — obligation from outside
Nigel has to approve every look before a Runway shoot in The Devil Wears Prada.
Andy doesn’t have to love the outfit Miranda picks — she just has to deliver it.
don’t/doesn’t have to = not necessaryDoes Andy have to answer Miranda’s calls at midnight?
Do/Does + subject + have to + base verbCan / Could — permission
Visitors to the Devil Wears Prada 2 set can take photos in the lobby, not in the studio.
Junior staff at Runway can’t leave before Miranda Priestly does.
Could Andy take Friday afternoon off from Runway?
Could is more polite/tentative than canWhen to use which
Obligation & prohibition
- Speaker’s own rule/opinion: A screenwriter working with Billy Wilder might say, “I must finish this script before Friday.”
- External rule, law or system: Drivers have to stop at a red light — exactly the kind of rule ignored during the famous car chase in Bullitt.
- Forbidden — never do it: You mustn’t record the screening of Dune: Part Two.
- Not necessary — free choice: You don’t have to stay for the credits of Avengers: Endgame, but most fans do.
Permission & choice
- Informal permission: “Can I sit here?” a fan asks at the Devil Wears Prada 2 premiere.
- Polite/tentative permission: “Could I ask a question, Mr Cruise?” a journalist says.
- Formal permission: “May I approach the bench?” — the kind of formal request a lawyer makes in a courtroom drama like A Few Good Men.
- Refusing permission: You can’t bring outside food into the cinema during a Marvel premiere.
Signal words: your instant clue
These words often travel alongside one meaning or the other — spotting them speeds up your choice.
Obligation signals
Permission & optional signals
Visual guide
Teacher Teddy’s illustrated guide to this rule is being drawn — the full swipe-through infographic will appear here soon.
Common mistakes
❌ You mustn’t come to the Devil Wears Prada 2 wrap party if you don’t want to.
✅ You don’t have to come to the Devil Wears Prada 2 wrap party if you don’t want to.
The rule isn’t forbidding you — it’s simply optional. Mustn’t would mean the opposite: banned.
❌ Miranda musts finish the Runway layout by Thursday.
✅ Miranda must finish the Runway layout by Thursday.
Modal verbs never take an -s, even with he/she/it.
❌ Does Andy must ask permission to leave the Devil Wears Prada set?
✅ Must Andy ask permission to leave the set? / Does Andy have to ask permission?
Must forms its own question without “do”; have to needs do/does because it behaves like an ordinary verb.
British vs American note: British English often prefers the informal have got to — I’ve got to go — where American English sticks with have to. Both mean the same, but “have got to” has no past form; you can’t say “had got to”.
Exceptions & tricky cases
Four traps worth knowing once the basic rule feels comfortable.
1. “Must” can mean a guess, not a rule. Out of context, must sometimes shows near-certainty rather than obligation: Miranda Priestly’s flight landed an hour ago — she must be at the Runway office by now. That’s a different job for “must” (covered fully in our lesson on modals of speculation).
2. Only “have to” changes with tense. Must has no past or future form, so English borrows “have to” instead: ✅ Andy had to reschedule the Devil Wears Prada 2 shoot. ❌ Andy musted reschedule the shoot.
3. “Needn’t” is a formal British twin of “don’t have to”. Nigel needn’t rush the Runway layout — the deadline moved. It’s rarer in speech but common in British films and older scripts.
4. “Can’t” for prohibition sounds softer than “mustn’t”. Both forbid something, but can’t often reports someone else’s rule (Guests can’t use their phones during a Cannes Film Festival screening), while mustn’t often carries the speaker’s own firm warning (You mustn’t tell anyone the twist ending of The Sixth Sense!).
Practice: choose the right modal
Eight quick questions about obligation and permission — pick the answer you’d actually say.
1. Passengers on the flight to the Cannes Film Festival ___ fasten their seatbelts before take-off — it’s the law.
A legal rule from outside the speaker → have to.
2. You ___ tell anyone the twist ending of The Sixth Sense!
Forbidden, not optional → mustn’t.
3. ___ I take a photo with you, Mr Cruise?
A polite request for permission → could.
4. Extras on the Devil Wears Prada 2 set ___ show up at 5 a.m., but they usually do anyway.
Optional, not forbidden → don’t have to.
5. ___ the director film on location in New York for Breakfast at Tiffany’s, or was it a studio rule?
Must has no past form — use did … have to.
6. Visitors to the Louvre in the Da Vinci Code tour ___ use flash photography in the gallery.
A prohibition on the visitor, reported as a rule → can’t.
7. “___ I approach the witness, Your Honour?” a lawyer asks formally, in the style of A Few Good Men.
Courtroom formality calls for the most polite option → may.
8. Cast members ___ sign a non-disclosure agreement before reading the Marvel script.
A studio requirement, not permission → have to.
Movie spotlight: The Devil Wears Prada 2
Few plots run on obligation and permission as tightly as this one: fashion’s most feared editor now has to ask her former assistant for a favour, decides what she mustn’t reveal about her position, and negotiates exactly what each side can and can’t demand from the other. Every power move in the office is one of these modals playing out on screen, which makes the film an unusually clear window into how English handles duty, rules and permission.
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