Modal Verbs: How to Use Can, Could, May, Might, Must, and More

Modal verbs are auxiliary verbs that express necessity, possibility, permission, ability, and obligation. Unlike regular verbs, modals don't change form for different subjects and are always followed by the base form of the main verb.

What Makes Modal Verbs Special

Modal verbs follow special rules. They do not take -s in the third person: He can swim (not he cans swim). They form negatives by adding not: cannot, should not, must not. They form questions by inverting with the subject: Can you help? They are followed by the base verb without to: She must go (not she must to go).

Can and Could

Can expresses ability in the present, permission, and possibility.

I can speak three languages. (ability)

Can I borrow your pen? (permission)

It can get very cold here. (possibility)

Could is the past form of can, used for past ability, polite requests, and less certain possibilities.

I could run fast when I was young. (past ability)

Could you help me with this? (polite request)

It could rain later. (less certain possibility)

May and Might

Both may and might express possibility, with may suggesting a higher probability. May is also used for formal permission.

It may rain tomorrow. (fairly likely)

It might rain tomorrow. (less likely)

You may leave early today. (formal permission)

Might is also used as the past form of may in reported speech: She said she might come.

Must and Have To

Must expresses strong obligation coming from the speaker. Have to expresses external obligation. In everyday English, have to is more common.

I must finish this today. (my own decision)

I have to wear a uniform at work. (external rule)

Must not (mustn't) means prohibition. Do not have to means lack of necessity.

You must not park here. (prohibited)

You don't have to wait. (not necessary)

Shall and Should

Shall is used mainly in formal English for offers, suggestions, and future time with first-person subjects.

Shall I open the window? (offer)

We shall arrive at noon. (formal future)

Should expresses advice, recommendation, and expectation.

You should see a doctor. (advice)

They should arrive soon. (expectation)

Ought to has the same meaning as should but is less common in modern English: You ought to apologize.

Will and Would

Will expresses future actions, promises, offers, and willingness.

I will call you tomorrow. (future)

I will help you. (promise)

Would is the past form of will. It expresses polite requests, hypothetical situations, past habits, and preferences.

Would you like some tea? (polite offer)

If I were rich, I would travel. (hypothetical)

When I was young, I would visit my grandmother every summer. (past habit)

Modal Perfect Forms

Modals can combine with have + past participle to refer to the past. These forms express speculation, criticism, or missed opportunities.

She must have forgotten. (certainty about past)

You should have told me. (past advice that wasn't followed)

He might have missed the train. (past possibility)

I could have been a doctor. (missed past ability)

Common Modal Verb Mistakes

Learners often use to after modals: I can to swim is incorrect. Another common error is using must for past obligation — use had to instead: Yesterday I had to work late (not must work). Also, do not use modals together: I will can go is wrong — say I will be able to go.

Improve Your Modal Verb Usage

Using modal verbs correctly adds precision to your writing. Use our free reading time calculator to estimate how long your text takes to read and check for natural modal verb usage.

Frequently Asked Questions

What is the difference between may and might?

May suggests a higher probability. Might suggests a lower probability. In everyday speech, the distinction is often ignored.

Can I use two modals together?

No. English does not allow two modals together. Instead, use modal + semi-modal: I will be able to come (not I will can come).

What is the past form of must?

Must does not have a past form. For past obligation, use had to. For past prohibition, use was not allowed to.

Is shall still used in modern English?

Yes, but mainly in formal English, legal documents, and British English for offers and suggestions. In American English, will is preferred.

What are modal perfect forms?

Modal perfect forms use modal + have + past participle to refer to the past: should have gone, must have been, could have done.