Modal Verbs (Can, Could, May, Might, Must): English Grammar Guide

Introduction

Modal verbs are special auxiliary verbs that express the speaker’s attitude toward the action or state described by the main verb. They show possibility, ability, permission, necessity, or obligation. Unlike regular verbs, modal verbs do not take -s in the third person singular, are followed by the base form of the main verb, and have no infinitive or past participle form.

In this guide, we will explore the most common modal verbs — can, could, may, might, and must — with examples and explanations that will help you use them naturally in English conversation and writing.

What Are Modal Verbs?

Modal verbs are used with the base form of a verb (the infinitive without “to”) to modify its meaning. For example:

• She can swim.

• You must study harder.

• It might rain tomorrow.

Each modal verb expresses a specific meaning related to ability, permission, possibility, necessity, or probability.

Modal Verb: CAN

1. Ability

Can expresses ability or skill in the present.

• I can speak Japanese.

• She can play the piano.

2. Permission

Can is used to ask for or give permission in informal contexts.

• Can I go out now?

• You can use my laptop.

3. Possibility

It also expresses that something is possible.

• It can be dangerous to drive at night.

Notes

• Negative form: cannot (can’t)

  • I can’t swim.

• Past ability uses could instead of can.

Modal Verb: COULD

1. Past Ability

Could is the past tense of can when talking about ability.

• When I was a child, I could climb trees easily.

2. Polite Requests or Suggestions

It makes requests or offers sound more polite.

• Could you help me, please?

• You could try restarting your computer.

3. Possibility

Could also expresses possibility, especially when something is uncertain.

• It could rain later.

• That could be the answer.

4. Hypothetical Situations

Used in conditional sentences or imagined situations.

• If I had more time, I could travel the world.

Modal Verb: MAY

1. Permission (Formal)

May is used to ask or give permission, especially in formal English.

• May I come in?

• You may leave early today.

2. Possibility

May expresses a real possibility in the present or future.

• It may snow tonight.

• She may know the answer.

3. Expressing Wishes

Sometimes may is used in formal expressions of good wishes.

• May you live a long and happy life.

Notes

• Negative form: may not

  • You may not enter this area.

Modal Verb: MIGHT

1. Weaker Possibility

Might shows a smaller chance or more uncertainty than may.

• It might rain later (less likely than “may rain”).

• She might be at the library.

2. Past Form of “May” in Reported Speech

Used when reporting speech in the past.

• She said she might come later.

3. Polite Suggestions or Hypotheticals

• You might want to check that again.

• If I had known, I might have helped you.

Notes

Might and may are often interchangeable, but might sounds more uncertain or tentative.

Modal Verb: MUST

1. Strong Necessity or Obligation

Must expresses strong necessity, duty, or moral obligation.

• You must wear a seatbelt.

• Students must submit their assignments on time.

2. Logical Deduction (Strong Probability)

Used when you are almost certain about something.

• He’s been working all day; he must be tired.

• You must be joking!

3. Rules and Instructions

• You must not smoke here.

• Visitors must sign in at reception.

Notes

• Negative form: must not (mustn’t)

  • You mustn’t touch that — it’s dangerous.

Comparison of Modal Verbs

Modal Verb Grammar Rules

1. No -s in the third person singular:

   • He can play, not he cans play.

2. Followed by the base form of a verb:

   • She might come. (not might comes)

3. Negatives:

   • You must not be late.

   • I can’t find my keys.

4. Questions:

   • Can you help me?

   • May I leave now?

5. No infinitive or past participle:

   • Incorrect: to can, has musted

   • Correct: be able to, had to (use alternatives for tenses).

Modal Verb Alternatives

When modal verbs cannot express the desired tense or meaning, we use phrases:

• Be able to (for ability):

  • I was able to finish the project on time.

• Have to (for necessity):

  • You have to wear a uniform.

• Be allowed to (for permission):

  • Students are allowed to use their phones during breaks.

Common Mistakes to Avoid

1. ❌ He cans swim. → ✅ He can swim.

2. ❌ I must to go now. → ✅ I must go now.

3. ❌ She can sings well. → ✅ She can sing well.

4. ❌ May not to enter. → ✅ May not enter.

Always remember: modal verbs are followed directly by the base form of the main verb — no to, no -s, and no -ed.

Practice Sentences

1. I can speak three languages.

2. You could try restarting your laptop.

3. It may rain this evening.

4. He might join us later.

5. You must wear a helmet when riding a motorbike.

Conclusion

Modal verbs like can, could, may, might, and must are essential tools in English communication. They help you express degrees of certainty, permission, ability, and obligation in a natural and concise way. Mastering them will make your English sound more fluent and confident.

To improve, practice identifying the function (possibility, necessity, permission, etc.) each time you see or use a modal verb. The more you notice them in real sentences — in conversations, movies, or articles — the more naturally you’ll use them yourself.

FAQs

What are modal verbs and why are they important?

Modal verbs are auxiliary verbs—such as can, could, may, might, and must—that modify the meaning of a main verb to express ability, permission, possibility, or necessity. They do not change form (no -s for third person singular), are followed by the base form of the verb, and lack infinitive or participle forms. Mastering modals helps you make precise statements about likelihood, rules, and obligations, which is essential for clear, natural English in speaking and writing.

How do I form sentences with modal verbs?

Use the structure subject + modal + base verb + complement. Do not add to before the main verb and do not conjugate the modal.

• She can swim (✔) vs. She can to swim (✘)
• They must leave now (✔) vs. They must leaves now (✘)

What is the difference between “can” and “could” for ability?

Can shows present ability, while could typically refers to general ability in the past. For one-time or achieved ability in the past, English often prefers was/were able to.

• Present: I can drive.
• Past (general): When I was ten, I could ride a bike.
• Past (specific success): I was able to fix the bug yesterday.

How do “may” and “might” differ in expressing possibility?

Both may and might indicate possibility. In modern usage, might often suggests a slightly weaker or more tentative possibility than may. In formal writing, some writers prefer may for neutral possibility and might for lower probability or hypothetical contexts, but in everyday conversation they can be interchangeable.

• Neutral possibility: It may rain later.
• Tentative possibility: It might rain, but the sky looks clear.

When should I use “must,” and how is it different from “have to”?

Must expresses strong internal necessity, rules set by the speaker, or logical deduction. Have to often indicates external requirements or circumstances. In many contexts they overlap, but must can sound stronger or more categorical.

• Internal or speaker-imposed rule: You must submit your work by noon.
• External obligation: I have to submit my taxes by April 15.
• Deduction: He must be tired after the trip.

How do I make negatives and questions with modal verbs?

For negatives, place not directly after the modal: cannot/can’t, may not, might not, must not (mustn’t). For questions, invert the subject and the modal.

• Negative: She might not join us.
• Negative (strong prohibition): You mustn’t smoke here.
• Question: Can you help?, May I sit here?

Is “may not” the same as “cannot” or “must not”?

No. May not typically expresses lack of permission or a possibility that something will not happen. Cannot expresses impossibility or inability. Must not expresses prohibition.

• You may not enter. (Permission denied)
• We cannot enter. (It’s impossible for us to enter)
• You must not enter. (Strict rule: do not enter)

How do modals express politeness (e.g., requests and offers)?

More tentative modals typically sound more polite. Could and might feel softer than can or may. In requests, could is widely used to add courtesy; may is formal for permission.

• Polite request: Could you email the report?
• Formal permission: May I speak with you?
• Informal permission: Can I borrow your pen?

Can I use “must” for deductions about the past?

Yes, but you need a perfect infinitive (have + past participle) after the modal to set the time in the past. For negative deduction in the past, use can’t/couldn’t have + past participle.

• Past deduction (almost sure): She must have left early.
• Past negative deduction: He can’t have seen the message.

How do I express obligation or necessity in different tenses?

Modals themselves do not inflect for tense. Use semi-modal alternatives to shift time:

• Present: must / have to — You must wear a helmet.
• Past: had to — We had to cancel the meeting.
• Future: will have to — You will have to sign the form tomorrow.

What are common mistakes with modal verbs?

Typical errors include adding -s to modals, inserting to before the base verb, and using the wrong negative form.

• ✘ He cans swim. → ✔ He can swim.
• ✘ I must to go. → ✔ I must go.
• ✘ She can sings. → ✔ She can sing.
• ✘ May not to enter. → ✔ May not enter.

How do I choose between “may” and “can” for permission?

In everyday conversation, can is widely used for permission, while may is considered more formal or traditional. In academic or professional settings, may might be preferred to avoid ambiguity with ability. In casual speech, Can I…? is perfectly acceptable.

Can I use “might” in the past or with conditionals?

Yes. Use might to express hypothetical or unreal situations, often with conditionals. For past hypotheticals, combine might with the perfect infinitive.

• Conditional: If you left now, you might catch the bus.
• Past hypothetical: If I had known, I might have helped.

What is the structure for perfect infinitives after modals (e.g., “must have done”)?

Use modal + have + past participle to refer to completed actions or states relative to a reference time. This is common with deduction, speculation, and unreal past scenarios.

• Deduction: They must have arrived by now.
• Speculation: She might have forgotten.
• Unreal past: We could have won if we’d tried.

How do I express logical certainty, probability, and possibility with modals?

English uses a scale of certainty with modals and adverbs. For strong certainty, use must for positive deduction and can’t/couldn’t for negative deduction. For probability and possibility, use should, may, might, or could depending on how confident you are.

• Near certainty: She must be at work.
• Strong negative deduction: He can’t be serious.
• Moderate probability: They should arrive soon.
• Open possibility: It could/may/might snow.

What are good alternatives when a modal doesn’t fit the tense I need?

Use semi-modals and periphrastic constructions:

• Ability across tenses: be able to — I will be able to join later.
• Necessity across tenses: have to / had to / will have to
• Permission across tenses: be allowed to — We were allowed to leave early.

How do I soften advice and suggestions using modals?

Use modals that signal tentativeness (could, might) or medium-strength recommendation (should). This makes advice polite and less prescriptive.

• Soft suggestion: You could try a smaller dataset.
• Gentle nudge: You might want to restart the app.
• Recommendation: You should back up your files.

Are there register (formality) differences among these modals?

Yes. May often sounds formal (especially for permission), while can is neutral and widely used in speech. Could is commonly used to add politeness. Must can sound strict or official. Choosing the right modal helps match your tone to the context—emails to supervisors may favor may and could, while casual chats favor can.

Can I stack modals (e.g., “might could”)?

Standard English does not stack modals (*might could, may can*). Some regional dialects do, but in formal and neutral contexts you should use a single modal or a modal plus a semi-modal (might be able to rather than *might can*).

How do modals behave in reported speech?

Many modals remain the same in reported speech (must, might), but others shift depending on the reporting context. Can often becomes could when reporting in the past. May commonly becomes might in reported statements.

• Direct: “I can help,” she said. → Reported: She said she could help.
• Direct: “I may go,” he said. → Reported: He said he might go.

What punctuation or word order rules should I remember with modals?

Word order follows the auxiliary pattern: modals precede the subject in questions and precede not in negatives. With adverbs, place them after the modal or before the main verb. Avoid double modals and maintain the base form of the main verb.

• Question: Could you explain this?
• Negative: We might not agree.
• Adverb placement: He might still finish on time.

Can I use contractions with modal verbs?

Yes, especially in informal writing and speech: can’t, couldn’t, mightn’t (less common in American English), mustn’t. Contractions make your tone more conversational, but avoid them in very formal documents or when clarity is paramount.

Practice: how can I quickly test my understanding?

Try transforming statements across functions and times:

• Ability → Permission: She can leave early. → May she leave early?
• Possibility → Deduction: He might be home. → He must be home; the lights are on.
• Present → Past obligation: We must finish today. → We had to finish yesterday.

If your sentence keeps the modal before the base verb, uses the correct negative form, and matches the intended degree of certainty or necessity, you’re using modals accurately.

English Grammar Guide: Complete Rules, Examples, and Tips for All Levels