Must is used when the speaker feels something is necessary or is giving an order; have to is used when the necessity comes from outside, from rules or circumstances. The difference is sharpest in the negative: mustn't means “it isn't allowed”, while don't have to means “it isn't necessary”. Must has no past form, so people say had to instead.
must or have to — can you pick the right one?
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must + verb (the speaker thinks it's necessary); have/has to + verb (rules, circumstances); mustn't + verb = not allowed; don't have to + verb = not necessary; had to (past)
- I must call my grandmother tonight; I haven't spoken to her for weeks.
- In our office we have to wear an ID badge, even on Fridays.
- You mustn't use your phone during the exam, or they'll ask you to leave.
What changed
First versionYou mustn't bring any food to the party because we've ordered pizza for everyone.
RevisionYou don't have to bring any food to the party because we've ordered pizza for everyone.
Mustn't looks like the simple negative of must, but it's a ban: You mustn't bring any food means that food is not allowed. The hosts are only telling their guest there's no need to bother, since the pizza is already ordered, so the right form is don't have to.
Must has a second meaning: a confident guess. You've been working since six, you must be exhausted means “I'm sure you're very tired”. There's no obligation here: the speaker is drawing a conclusion from what they can see.
Your turn
The lift was broken yesterday, so I ___ walk up to the tenth floor.
Show the answer
had to — The sentence is about yesterday, and must has no past form. To say what you were obliged to do in the past, use had to. Must refers to the present or the future and doesn't go with yesterday.
Describe the rules at your work or place of study. Name one thing you are required to do and one thing that is not necessary.
At my college we have to hand in essays online, but we don't have to print them.
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