Overview
Advanced obligation modal distinctions include: must vs. have to (internal vs. external obligation), need not vs. must not (no obligation vs. prohibition), ought to (moral obligation), be supposed to (expectation), and had better (strong advice with a warning tone).
Obligation Distinctions
- mustmust — internal obligation / speaker's strong insistence
- have tohave to — external obligation / rules / circumstances
- need toneed to — necessity (can be subjective)
- ought toought to — moral duty / advice (weaker than must)
Negative Forms
- mustn't — prohibition (you are not allowed to)
- don't have to — no obligation (it is not necessary)
- needn't — lack of necessity (= don't need to)
- ⚠️ mustn't ≠ don't have to: very different meanings!
Common Mistakes
- ❌ You must not wear a tie (if informal) → ✅ you don't have to
- ❌ He must to go → ✅ He must go (no 'to' after must)
- ❌ Past of must (obligation): musted → ✅ had to
When to use
Rules
You must show your passport at the border.
Workplace
She has to attend the weekly meeting on Fridays.
Prohibition
You mustn't use your phone during the exam.
No obligation
You don't have to bring anything — it's provided.