Modal Verbs: Can, Should, Must, and Have To

Modal verbs let you say more than what happened — they let you say what's possible, necessary, allowed, or advisable. There are only a handful, and they all follow the same simple grammar.

The shared grammar

Every modal works the same way:

• No -s in the third person: She can swim. (not "cans")
• Followed by the base verb, with no to: You should go. (not "should to go")
• Negatives and questions need no do: Can you help? / I can't come.

The one exception is have to, which behaves like a normal verb: She has to go. Does she have to go?

Ability: can / could

• I can speak three languages. (present)
• I could swim when I was five. (past ability)

Permission: can / may

• Can I borrow your pen? (normal, everyday)
• May I borrow your pen? (formal and polite)

Advice: should / ought to

• You should see a doctor.
• You shouldn't work so late.

Should is the everyday word for advice — friendly, not forceful.

Obligation: must vs. have to

This is the distinction learners most often get wrong.

	Meaning	Example
must	The speaker's own authority or strong feeling	I must call my mother — I keep forgetting.
have to	An outside rule or circumstance	I have to wear a uniform at work.

In practice, have to is far more common in speech. Use must for rules you're imposing yourself, and in written notices: Passengers must not smoke.

The negative trap

Here the two split completely — this catches almost everyone:

• You mustn't park here. → It is forbidden.
• You don't have to park here. → It is not necessary (you may if you want).

Mustn't is a prohibition. Don't have to is an absence of obligation. They are close to opposites.

Possibility: might / may / could

• It might rain later. (possible, uncertain)
• She could be at home. (possible)

All three express roughly the same uncertainty at B1 level.

Try it yourself

The quiz below focuses on the distinctions that actually cause errors — especially mustn't vs. don't have to.