Obligation: must and have to
Both must and have to express obligation — something that is necessary.
| Verb | Source of obligation | Example |
|---|---|---|
| must | internal / the speaker decides it's necessary | I must call my mum — it's her birthday! · You must try this cake. |
| have to | external / a rule, law, or other person requires it | I have to wear a uniform at work. · She has to be home by 10. |
In everyday speech:
"Must" and "have to" often feel the same. "Have to" is more common in everyday conversation. "Must" sounds stronger or more urgent. In the past, only "had to" exists — "musted" is not a word.
No obligation: don't have to
Don't have to / doesn't have to means something is not necessary — you have a choice. There is NO obligation.
You don't have to come if you don't want to. (= it's OK if you stay)
She doesn't have to work on Sundays. (= Sundays are free)
I don't have to cook tonight — we're ordering pizza.
She doesn't have to work on Sundays. (= Sundays are free)
I don't have to cook tonight — we're ordering pizza.
Prohibition: mustn't
Mustn't means something is forbidden or not allowed. It is much stronger than "don't have to".
You mustn't smoke in here. (= it's forbidden)
Students mustn't use their phones during the exam.
You mustn't park here — it's a fire exit.
Students mustn't use their phones during the exam.
You mustn't park here — it's a fire exit.
Don't have to ≠ Mustn't — crucial difference!
"You don't have to wear a tie." = You can wear one if you like, but it's not required. "You mustn't wear a tie." = Ties are forbidden here. One means no obligation, the other means prohibition.
| Expression | Meaning |
|---|---|
| must / have to | obligation — it is necessary |
| don't have to | no obligation — it is not necessary (but possible) |
| mustn't | prohibition — it is forbidden / not allowed |
Exercises
Choose the correct modal verb.
1. In this country, children ___ go to school until they are 16. It's the law.
2. You ___ bring food to the party — I'm making everything myself.
3. You ___ touch that wire — it's live and very dangerous!
4. I ___ remember to post this letter — it's really important.
5. She ___ speak Spanish at work. Her boss prefers English.