Modal Verbs Can, Could, May, Might and Must: Ability, Permission, Possibility and Obligation

Can, could, may, might and must are modal verbs: helping verbs that add meaning to a main verb. Use can and could for ability (I can swim); can, could and may for permission (Can I sit here? May I come in?); may, might and could for possibility (It might rain); and must for strong obligation or a confident conclusion (You must wear a seatbelt. She must be tired). Modal verbs follow simple grammar rules: they never change form, they are followed by the base form of a verb without to, and they make questions and negatives without do. This guide explains each meaning with clear examples, compares the verbs in tables, shows how to talk about the past, and ends with a practice exercise.

What modal verbs are and their grammar rules

A modal verb shows the speaker's attitude to an action: whether it is possible, allowed, necessary or certain. The Cambridge Grammar of English lists the core modals as can, could, may, might, will, would, shall, should, must. This guide covers five of them.

All five follow the same rules:

Rule	Correct	Wrong
No -s with he, she, it	She can drive.	She cans drive.
Base verb without to	You must go.	You must to go.
No do in questions	Can you help me?	Do you can help me?
Add not for negatives	I can't swim. You mustn't park here.	I don't can swim.
Only one modal at a time	I will be able to come.	I will can come.
No -ing or to- form	I'd like to be able to cook.	I'd like to can cook.

Contractions: can't (or cannot, written as one word), couldn't, mightn't (rare; most people say might not), mustn't. May not is not normally contracted.

Ability: can and could

Use can for ability in the present and could for general ability in the past.

• I can speak three languages.
• Can you read this sign from here?
• She couldn't swim when she was a child, but she can now.
• My grandfather could fix anything.

Could vs was able to

For a single success on one occasion in the past, use was able to or managed to, not could:

• Correct: The exam was difficult, but I was able to finish it.
• Unnatural: The exam was difficult, but I could finish it.
• In the negative, couldn't is fine for one occasion: I couldn't finish the exam.

Other tenses: be able to

Can has no future, perfect or infinitive form, so use be able to: I'll be able to help you tomorrow. I've never been able to sing. I want to be able to speak English at work.

Permission: can, could and may

All three ask for or give permission. The difference is mainly formality.

Verb	Formality	Asking	Giving / refusing
can	Neutral, informal; most common in speech	Can I open the window?	Yes, you can. / No, you can't.
could	More polite; good with strangers and at work	Could I ask you a question?	Answer with can: Yes, of course you can. (not Yes, you could)
may	Formal; used in official rules and very polite requests	May I come in?	You may leave when you have finished. Visitors may not take photographs.

Note that could in Could I...? is not past; it is simply more polite. For polite requests that ask someone else to do something, use Can you...? or Could you...? (Could you send me the file?), not May you. Our guides to writing work emails and formal letters show these phrases in context.

For past permission, use could or was allowed to: When I was young, I could stay up late at weekends. For one specific occasion, prefer was allowed to: I was allowed to leave early yesterday.

Possibility: may, might, could and can

Use may, might and could when something is possible but not certain, now or in the future. In most everyday situations they mean almost the same thing.

• Take an umbrella. It might rain.
• The meeting may finish late.
• "Where's Tom?" "He could be in the library."

Verb	Typical feeling	Example
may	Possible; slightly more formal	Prices may increase next year.
might	Possible; very common in speech; can sound a little less likely	I might go to the party, I'm not sure.
could	Possible; often one option among others	We could be late if the traffic is bad.
can	General, theoretical possibility (things that sometimes happen)	English spelling can be confusing.

Negative possibility: a common trap

The negatives are not all the same:

• He may not / might not come. = Perhaps he will not come.
• He can't / couldn't be at home. = I am sure he is not at home (see deduction below).

Do not use can't to mean "perhaps not". It can't rain today means you are certain it will not rain.

Questions about possibility

In questions, use might, could or a phrase with likely, not may: Do you think it might rain? Could this be the answer? Is he likely to come?

Obligation: must and have to

Use must for strong obligation or necessity, especially when the obligation comes from the speaker, or in written rules.

• I must call my mother tonight. (I feel it is necessary.)
• You must wear a seatbelt.
• Passengers must show their boarding pass. (Written rule.)

Have to has a similar meaning and is more common in everyday speech, especially for obligations that come from outside (laws, rules, other people): I have to work on Saturday. Do you have to wear a uniform?

Mustn't vs don't have to

This is one of the most important differences for learners, because the meanings are very different:

Form	Meaning	Example
mustn't	It is not allowed. Don't do it.	You mustn't smoke in the building.
don't have to	It is not necessary. You can if you want.	You don't have to come to the meeting. It's optional.

Past obligation

Must has no past form for obligation. Use had to: I had to work late yesterday. (not I must work late yesterday)

Deduction: must, might and can't for being sure

Modal verbs also show how sure you are about something, based on evidence. This is called deduction or speculation.

How sure?	Present	Past (modal + have + past participle)
Almost certain: yes	She must be tired. She's worked twelve hours.	She must have been tired.
Possible	He might / may / could be in a meeting.	He might / may / could have missed the train.
Almost certain: no	That can't be Anna. She's in Paris.	That can't / couldn't have been Anna.

Notice that the opposite of must in deduction is can't, not mustn't. He mustn't be at home is wrong for deduction; say He can't be at home.

Talking about the past: a summary

Meaning	Present	Past
General ability	can	could
Ability on one occasion	can	was able to / managed to (negative: couldn't)
Permission	can / may	could / was allowed to
Obligation	must / have to	had to
Possibility about the past	may / might / could	may / might / could have + past participle
Deduction about the past	must / can't	must have / can't have + past participle
Criticism or missed chance	-	You could have told me! (You didn't, and you should have.)

For more on choosing past tenses, see our guides to English tenses and present perfect vs past simple.

The most common learner mistakes

1. Adding to: I must to study. Fix: I must study.
2. Adding -s: He cans play the guitar. Fix: He can play.
3. Using do: Do you can help me? Fix: Can you help me?
4. Two modals together: I will can go. Fix: I will be able to go.
5. "Must" for past obligation: Yesterday I must leave early. Fix: Yesterday I had to leave early.
6. Confusing mustn't and don't have to: You mustn't pay, it's free. Fix: You don't have to pay.
7. "Could" for one past success: After three tries I could pass my driving test. Fix: I was able to / managed to pass.
8. "Mustn't" for negative deduction: It mustn't be true. Fix: It can't be true.

Several of these are in our list of common English grammar mistakes.

Practice: choose the best modal

Choose the best option. Answers are below.

1. When I was five, I (can / could) read.
2. (May / Must) I use your phone, please?
3. Take a jacket. It (might / must) get cold later.
4. You (mustn't / don't have to) bring food. Lunch is provided.
5. You (mustn't / don't have to) touch that wire. It's dangerous.
6. She has three jobs. She (must / can't) be very busy.
7. That (can't / mustn't) be the right address. It's a park.
8. The lift was broken, so we (must / had to) walk up.
9. After an hour, we (could / were able to) find a parking space.
10. I'm not sure where Sam is. He (could / must) be at the gym, or at home.

Answers

1 could. 2 May. 3 might. 4 don't have to. 5 mustn't. 6 must. 7 can't. 8 had to. 9 were able to. 10 could.

How to practise modal verbs

• Speak about real plans. Each evening, say three sentences about tomorrow with might, have to and can: I have to finish a report. I might go to the gym. I can't meet Ana.
• Play "detective". Look at a photo and make deductions: She must be a doctor. They can't be on holiday. It might be winter.
• Collect polite requests. Notice how colleagues ask for things in emails and meetings (Could you...? Would it be possible to...?) and copy the phrases.
• Get feedback on speaking. Modals are easy to understand but hard to use quickly. Practise with a partner, or see our guide to practising speaking without a partner. Our courses and free lessons give structured practice.

What this article cannot tell you

• Every modal meaning. We cover the main uses of five modals. Should, would, will, shall and semi-modals such as ought to and need to have their own uses.
• All differences between may, might and could. In possibility, the difference is often small and depends on the speaker, region and context. Many native speakers use them interchangeably.
• Regional variation. Mustn't and shall are more common in British English; American speakers often prefer can't or don't for prohibition (You can't park here).
• Your level. Basic ability and permission are usually taught at CEFR A1–A2; deduction about the past is usually B2. Check your level with our English level test.