Overview
Modal verbs for obligation: must, have to, need to. For prohibition: must not, cannot, may not. For advice: should, ought to, had better. Each carries a different level of strength — from strong obligation (must) to mild advice (should). Understanding these distinctions is essential for formal communication.
Obligation / Prohibition / Advice
- obligationmust / have to — You must show ID. / Staff have to wear badges.
- prohibitionmustn't — You mustn't use flash photography here.
- adviceshould / ought to — You should see a doctor.
- no obligationdon't have to — You don't have to attend — it is optional.
Key distinctions
- Mustn't ≠ don't have to — forbidden vs. optional!
- Must = personal strong rule. Have to = external rule.
- Had better + base verb = strong advice with warning: You'd better leave now.
- Ought to = slightly formal version of 'should'.
Common Mistakes
- ❌ You mustn't to attend. → ✅ You don't have to attend. (no obligation)
- ❌ He ought to called. → ✅ He ought to call. (no -ed after 'to')
- ❌ You had better to leave. → ✅ You had better leave. (no 'to')
When to use
Health & safety
Visitors must not touch the exhibits.
Workplace rules
All staff have to complete the training.
Academic advice
You should proofread your essay carefully.
Strong warnings
You'd better book now before it sells out.