Modal Verbs: A Complete Guide to Can, Could, Must and More
Clear rules for can, could, may, might, must, shall, should, will, would. Quick-reference table: permission, obligation, probability. Practice exercises inside.
Modal Verbs: A Complete Guide to Can, Could, Must and More
Learn how to use all nine modal verbs in English: can, could, may, might, must, shall, should, will, and would. Includes a quick-reference permission table, common mistakes, and practice exercises.
Quick-Reference Table: Permission, Obligation and Probability
This table shows how each modal verb works across three common uses. Use it as a quick cheat sheet when you are not sure which modal to choose.
| Modal Verb | Permission | Obligation | Probability |
|---|---|---|---|
| Can | You can go now (informal) | - | It can be true (general possibility) |
| Could | Could I borrow this? (polite) | - | It could rain later (50% chance) |
| May | You may enter (formal) | - | It may snow tonight (50% chance) |
| Might | Might I ask a question? (very polite) | - | It might be too late (30% chance) |
| Must | - | You must wear a seatbelt (strong obligation) | She must be home (95% certainty) |
| Shall | Shall I open the window? (suggestion) | You shall not pass (formal prohibition) | - |
| Should | - | You should see a doctor (advice, weak obligation) | They should be here soon (80% expectation) |
| Will | - | I will call you (promise, strong intention) | It will rain today (100% prediction) |
| Would | Would you like tea? (polite offer) | - | It would be nice (hypothetical) |
What Are Modal Verbs?
Modal verbs are auxiliary (helping) verbs that express necessity, possibility, permission, ability, or obligation. They modify the meaning of the main verb that follows them. The nine main modal verbs in English are: can, could, may, might, must, shall, should, will, would.
Modals have special grammar rules that make them different from normal verbs. They do not change form for third person (no -s). They are always followed by the base form of the verb (no "to" after the modal). And they do not have an -ing form or a past participle.
Rule 1: No -s in third person: He can swim (not "he cans swim")
Rule 2: No "to" after modal: She must go (not "she must to go")
Rule 3: Base verb after modal: I will call (not "I will calling")
Rule 4: No do/does in questions: Can you help? (not "Do you can help?")
Modal Verbs for Ability: Can, Could and Be Able To
Use these modals to talk about what someone can do, either now, in the past, or in a specific situation.
Can - Present Ability
I can speak three languages.
She can run five kilometres.
Can you play the guitar?
Could - Past General Ability
I could swim when I was five.
She could read before she started school.
We could not find the restaurant.
Be Able To - Specific Achievement
I was able to finish the report before the deadline.
She will be able to join us next week.
They have not been able to contact him.
Use could for general past ability ("I could run fast when I was young"), but use was/were able to for one specific success in the past ("I was able to catch the train"). For negative sentences, couldn't works for both general and specific situations.
Modal Verbs for Permission: Can, Could, May and Might
English gives you four options when asking or giving permission. The choice depends on how formal you want to sound.
Permission Scale (Least Formal to Most Formal)
Can (Informal)
Can I use your phone?
You can sit anywhere.
Could (Polite)
Could I borrow your pen?
Could we leave early today?
May (Formal)
May I come in?
You may begin the exam now.
Might (Very Formal)
Might I make a suggestion?
Might we ask for your help?
In everyday conversation, can is the most common choice for permission. May is more formal and common in written rules and official contexts. Use could to sound more polite, and might for very formal situations.
Modal Verbs for Obligation: Must, Have To, Should and Ought To
These modals express different levels of necessity and advice. The key difference is whether the obligation comes from inside yourself or from an external rule.
Must vs Have To
Must = internal obligation (you feel it is necessary)
I must exercise more. My health depends on it.
You must see that film. It is amazing.
Have to = external obligation (a rule or someone else requires it)
I have to wear a helmet at work.
She has to finish the report by Friday.
Must Not vs Don't Have To (Common Confusion)
Must not (mustn't) = prohibited, not allowed
You must not park here. It is a fire zone.
You must not use your phone during the exam.
Don't have to = not required, optional
You don't have to come if you are busy.
She doesn't have to wear a uniform at work.
Should vs Ought To (Advice)
You should drink more water.
You ought to apologise to her.
Both mean the same thing, but ought to is slightly more formal and less common in everyday speech. Should is used more often in conversation.
Modal Verbs for Probability: Must, Might, May, Could and Can't
Use these modals to show how sure you are about something. Each modal sits at a different level of certainty.
Probability Ladder
Must and can't are opposites when talking about probability. That must be the postman (I am almost sure) vs That can't be the postman (I am almost sure it is not). Use may and might when you are less certain.
Will vs Would and Shall vs Should
These four modals often cause confusion because they come in pairs with related meanings.
Will vs Would
Will = certainty, prediction, decision
I will call you tomorrow. (promise)
It will rain this weekend. (prediction)
I will have the steak, please. (decision at the moment)
Would = conditional, polite, hypothetical
I would help if I could. (conditional)
Would you like some coffee? (polite offer)
I would travel more if I had money. (hypothetical)
Shall vs Should
Shall = suggestions and first-person questions (mainly UK English)
Shall we go for a walk?
I shall be there at 8am. (formal promise)
Should = advice, expectation, weak obligation
You should try this restaurant. (advice)
The package should arrive today. (expectation)
The common question: Shall I... asks for instruction ("Shall I open the door?"), while Should I... asks for advice ("Should I accept the job?").
Practice Modals with Native Tutors on Preply
The best way to master modal verbs is to use them in real conversation. Preply connects you with native English tutors who can correct your modal usage in real time. Lessons start at $10/hour.
Find a Tutor on PreplyModal Verbs in Past Tense: Modal + Have + Past Participle
To use modals in the past, add have + the past participle of the main verb. This structure changes the meaning from present/ future to past.
Could have
I could have taken the earlier train.
(past ability that did not happen)
Should have
You should have told me earlier.
(past advice that was not followed)
Would have
She would have helped if you asked.
(past hypothetical result)
Must have
He must have forgotten the meeting.
(past deduction with high certainty)
Might have
They might have missed the flight.
(past possibility, low certainty)
Can't have
She can't have left already.
(past negative deduction)
Notice the difference in meaning: I should have studied (I did not study, and now I regret it) vs I could have studied (I had the ability but chose not to). Both express regret, but the emphasis is different.
Common Mistakes with Modal Verbs
These are the six most frequent errors that English learners make with modal verbs. Each one breaks one of the core grammar rules.
He can to swim very fast.
He can swim very fast.
Never put "to" after a modal verb. Use the base form directly.
She musts wake up early every day.
She must wake up early every day.
Modal verbs never take the third-person -s ending.
I can swimming in the ocean.
I can swim in the ocean.
Always use the base form (infinitive without "to") after a modal, not the -ing form.
You mustn't to go there alone.
You mustn't go there alone.
The negative form of a modal still uses the base verb, not "to" + infinitive.
He can plays the guitar very well.
He can play the guitar very well.
The main verb after a modal stays in the base form, no matter the subject.
May I might borrow your pen? (both in same sentence)
May I borrow your pen? or Might I borrow your pen?
Do not use two modal verbs together. Choose one that matches the formality and probability level you need.
Practice Exercises
Test your understanding of modal verbs. Choose the correct modal for each sentence based on the context given.
1. You look sick. You _____ see a doctor.
Answer: should (advice for a health concern)
2. _____ I open the window? It is very hot in here.
Answer: Shall (suggestion in UK English) or Can/Could (polite request)
3. She _____ be at work. Her car is in the parking lot.
Answer: must (strong deduction based on evidence)
4. I _____ speak Japanese when I lived in Tokyo, but now I have forgotten most of it.
Answer: could (past ability)
5. You _____ park here. There is a sign that says "No Parking."
Answer: must not / mustn't (prohibition)
6. We _____ go to the party tonight. We have not decided yet.
Answer: might or may (possibility, not yet decided)
7. I _____ studied harder for the exam. I failed by two points.
Answer: should have (past regret)
8. You _____ finish the report today. Tomorrow morning is fine.
Answer: don't have to (no obligation / optional)
9. Rewrite using a modal: "It is possible that she will arrive late."
Answer: She may arrive late. / She might arrive late.
10. Rewrite using a modal: "I am able to drive."
Answer: I can drive.
Frequently Asked Questions About Modal Verbs
What are the 9 main modal verbs?
The nine main modal verbs in English are: can, could, may, might, must, shall, should, will, and would. They are auxiliary verbs that express ability, permission, obligation, probability, and necessity. Learn more about how they work with our guide to conditional sentences, which uses many of these modals.
What is the difference between "may" and "might"?
Both express possibility, but may suggests a higher chance (around 50%) while might suggests a lower chance (around 30%). May is also used for formal permission: "You may enter." In everyday conversation, the difference is small and many speakers use them interchangeably.
When to use "must" vs "have to"?
Must is for internal obligation: "I must exercise more (I feel it is important)." Have to is for external rules: "I have to wear a uniform (my employer requires it)." In negative, must not = prohibited, don't have to = optional.
Can you use two modals together?
No. You cannot say "I will can come." Instead, use alternative structures: "I will be able to come" or "I might have to leave." For more on the difference between future forms, see our Will vs Going To guide.
What is the past form of modals?
Modal verbs use modal + have + past participle for the past: could have, should have, would have, must have, might have. For example: "I should have studied" (past advice), "She must have forgotten" (past deduction). Find more past tense patterns in our Past Tense guide.
Do modals change for third person?
Never. You say "he can," "she must," "it will" - no -s ending ever. This is one of the easiest grammar rules in English. Compare with present perfect vs past simple, where verb forms do change by subject.
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