Overview
Modal auxiliary verbs — can, could, may, might, must, shall, should, will, would — modify the main verb to express ability, possibility, permission, obligation, or prediction. They are followed by the base form of the verb and do not take -s in the third person singular.
Key Modal Verbs
- Abilitycan / could — I can swim. She could play.
- Permissionmay / can — May I leave? Can I help?
- Obligationmust / have to / should — You must stop.
- Possibilitymay / might / could — It might rain.
Grammar Rules
- Modal + base verb (never to/3rd-person -s)
- Negative: modal + not → cannot, shouldn't
- Question: modal + subject + base verb?
- No modal after another modal — use infinitive: I hope to be able to go
Common Mistakes
- ❌ She cans drive → ✅ She can drive
- ❌ You must to go → ✅ You must go
- ❌ He mights come → ✅ He might come
When to use
Ability
She can speak three languages.
Obligation
You must wear a seatbelt.
Permission
May I use your phone?
Possibility
It might snow tomorrow.