Modal verbs (also called modal auxiliaries) are special verbs that express necessity, possibility, permission, or ability. They do not change form for the third person and are always followed by the base form of the main verb.
The Modal Verbs at a Glance
| Modal | Meaning | Example | | ------------------------- | ----------------------------- | ---------------------------------- | | can | ability / permission | She can speak three languages. | | could | past ability / polite request | I could swim when I was five. | | should / ought to | advice / recommendation | You should see a doctor. | | must | strong obligation | You must wear a seatbelt. | | have to | necessity (external) | I have to finish this report. | | may | possibility / permission | May I leave early? | | might | possibility (less certain) | It might rain later. | | will | intention / future | I will call you tomorrow. | | would | polite request / hypothetical | Would you pass the salt? |
Key Grammar Rules
- No -s in the third person singular: He can swim (not he cans).
- Followed by the base verb: You must go (not you must to go).
- Questions invert the modal and subject: Can she come? / Should we leave?
Ability and Permission
- Can — present ability or informal permission: I can drive.
- Could — past ability or polite permission: When I was young, I could run fast.
- May — formal permission: You may begin the test.
Obligation and Necessity
- Must — strong internal obligation: You must pay taxes.
- Have to — external obligation (rules, others): I have to wear a uniform at work.
- Must not / Don't have to — prohibition vs. no obligation: You must not smoke here vs. You don't have to come early.
Advice
- Should — general advice: You should drink more water.
- Ought to — slightly stronger, less common: You ought to apologize.
- Had better — strong advice (with consequences): You had better hurry.
Possibility
- May — 50% possibility: She may be at the office.
- Might — 30% possibility: It might snow tomorrow.
- Could — general possibility: The weather could change.
Past Forms of Modals
| Present | Past | Example | | ----------- | ----------------------------- | ------------------------------------- | | can | could | I could run fast when I was 10. | | must | had to | I had to wake up early yesterday. | | should | should have + past participle | You should have told me. | | may / might | might have + past participle | She might have forgotten. |
Common Mistakes
- Using to after a modal: ~~She can to sing.~~ → She can sing.
- Adding -s to a modal: ~~He musts go.~~ → He must go.
- Confusing mustn't (prohibition) with don't have to (no obligation).