Have to, Must, and Their Negatives
We use have to and must to talk about obligation — things that are necessary. But their negative forms have very different meanings, so be careful!
Structure
- Have to: Subject + have to/has to + base verb. Example: I have to work on Saturday.
- Must: Subject + must + base verb (same for all subjects, no 's'). Example: You must wear a seatbelt.
- Negative: don't have to / doesn't have to (no obligation) vs mustn't (prohibition). Example: You don't have to come. vs You mustn't smoke here.
How the Grammar Works
- Use have to / has to for obligations that come from outside (rules, other people, situations): 'I have to renew my passport.'
- Use must for obligations the speaker feels personally, or strong rules: 'I must call my mother today.'
- Don't have to means there is no obligation — it's your choice: 'You don't have to pay, it's free.'
- Mustn't means it is forbidden — you must NOT do it: 'You mustn't park here.'
Common Use Cases
- Rules and laws: You must wear a helmet.
- Personal obligations: I have to finish this report today.
- Saying something is optional: You don't have to bring anything.
- Saying something is forbidden: You mustn't touch that.
Frequent Mistakes and How to Avoid Them
- Confusing 'don't have to' and 'mustn't': Incorrect: "You mustn't come if you're busy" (meaning it's optional). Correct: "You don't have to come if you're busy."
- Adding 's' to 'must': Incorrect: "She musts finish it." Correct: "She must finish it."
- Using 'to' after 'must': Incorrect: "You must to go." Correct: "You must go."
Comparison with Related Grammar
'Must' is stronger and more personal than 'have to', which often comes from outside rules. But the biggest difference is in the negative: 'don't have to' = no obligation (free choice), 'mustn't' = obligation NOT to do something (forbidden). These are opposite meanings, not just different words.
Example Sentences
- We have to leave early tomorrow.
- You mustn't use your phone during the exam.
- You don't have to answer if you don't want to.
- Employees must wash their hands before work.
- Does he have to wear a uniform?
When to Use This Rule
Use 'have to' and 'must' for obligation. Remember the key difference in the negative: 'don't have to' means it's not necessary, 'mustn't' means it's forbidden.