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)

For one specific past achievement, use was/were able to or managed to instead:

• ❌ The exam was hard but I could pass.
• ✅ The exam was hard but I was able to pass.

Could describes a general past ability; a single successful occasion needs the other form.

Permission: can / may

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

Requests and offers

Function	Least formal → most formal
asking someone	Can you…? · Could you…? · Would you mind…?
asking for yourself	Can I…? · Could I…? · May I…?
offering	Shall I…? · Would you like me to…?

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. Ought to means the same but is rarer and slightly more formal. For stronger advice: You'd better see a doctor. (implies a bad outcome if you don't)

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.

Note also that must has no past or future form — have to covers those: I had to work late. · I'll have to ask.

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. Needn't sides with don't have to: You needn't wait.

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.

Modals for deduction: how sure are you?

The same words do a second job — saying how confident you are about something now.

Modal	Certainty	Example
must	almost certain it's true	He must be tired — he's been up since five.
might / may / could	possible	She might be in a meeting.
can't	almost certain it's false	That can't be right — I checked it twice.

Note that the opposite of deductive must is can't, never mustn't.

And the past versions

Add have + participle to talk about deduction in the past:

• He must have missed the train.
• She might have forgotten.
• You can't have seen him — he's abroad.
• I should have asked first. (regret about the past)

One form, two jobs — must is either obligation (you must wear a helmet) or deduction (he must be exhausted). Context does all the disambiguating, so read the situation, not the word.

Modals doing both jobs at once

A: Do I have to book in advance?

B: You don't have to, but you should — it can get busy.

A: It's Tuesday. It can't be that bad, surely?

B: You'd be surprised. They must have had a review somewhere.

Have to and should are obligation and advice; can't and must have are deduction.

Try it yourself

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