Modal verbsEnglish level A2
Must / Mustn't
Use it for a strong obligation, a rule, or the speaker's certainty.
Rule
Must means strong obligation or a rule. Mustn't means forbidden, not lack of obligation. Past → had to; future → will have to.
Formula
| Positive (+) | Subject + must + V1 | You must finish this report.I must call her. |
|---|---|---|
| Negative (-) | Subject + mustn't + V1 | You mustn't smoke here. (forbidden!) |
| Question (?) | Must + subject + V1? | Must I do this? |
Compare
| must | have to |
|---|---|
| The speaker decides: I must finish this today. | A rule or situation decides: I have to wear a uniform. |
| Mustn't = forbidden: You mustn't smoke here. | Don't have to = not necessary: You don't have to come. |
Notes
- No direct past/future forms
- Past → had to
- Future → will have to
- mustn't is not don't have to: forbidden is not the same as not necessary
- Certainty about the present: You must be tired after such a long day.
- Questions with must are rare and formal — everyday English asks Do I have to...?
Practice
You missing word smoke here - it's forbidden.
The past of must: I missing word work last Saturday.