Modal Verbs: can, must, should, may, might (the "Force" Words)

This post is about a small but mighty group of helping words, the modal verbs: can, could, will, would, shall, should, may, might, must. They add shades of meaning to a verb: ability, permission, obligation, possibility, advice. With them you can say not just what happens, but how certain, how necessary, or how possible it is, and they all obey one simple structural rule that makes them easy to use.

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1. What is a modal verb?

A modal verb (or "modal" for short) is a special helping word that you put before a main verb to add a layer of meaning: meaning about ability, possibility, permission, obligation, or advice.

Compare:

• She swims. → just a plain fact.
• She can swim. → adds ability (she has the skill).
• She must swim. → adds obligation (she's required to).
• She might swim. → adds possibility (maybe she will).

See how the modal changes the flavor without changing the basic action (swim)? The modal adds the force behind the verb: how able, how required, how possible. That's why we can call them the "force words."

There are nine main modals:

can, could, will, would, shall, should, may, might, must

Quick recap: A modal verb (can, could, will, would, shall, should, may, might, must) goes before a main verb to add meaning: ability, possibility, permission, obligation, or advice. She can swim (ability), She must swim (obligation), She might swim (possibility).

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2. The one structural rule: modal + BASE verb

Here's the wonderful thing, all nine modals follow exactly one structural rule:

A modal is always followed by the BASE form of the verb (the plain verb: no -s, no -ed, no -ing, no to).

So:

• ✓ She can run.
• ✗ She can runs. (no -s)
• ✗ She can running. (no -ing)
• ✗ She can to run. (no to)

The base form is just the simplest version of the verb: run, go, be, submit, help. After any modal, that's what you use. This one rule covers all nine modals, that's what makes them structurally easy.

The formula:

Subject + Modal + base verb (+ rest)
She      + can   + run
You      + must  + submit  the form

Modals in the passive

If you want a modal with a passive ("done-to") verb, the formula adds be + past participle (remember the Boot and passives from Reading Time and Voice Off the Page: The Tense Matrix):

Subject + Modal + be + past participle

• The report can be submitted online.
• The machine should be repaired.
• The decision must be made today.

So modal + be + P.P. makes a passive modal. (You can see the Boot word be doing its job here, as always.)

Worth repeating: The one rule: a modal is always followed by the BASE verb (run, go, submit: no -s, -ed, -ing, or to). She can run. For passive: modal + be + past participle (The report can be submitted).

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3. The nine modals and what they mean

Now let's meet each modal's meaning. Many modals have more than one use, so I'll give the main ones:

Don't try to memorize all of these at once! The book's advice is wise: learn each modal with two or three example sentences covering its main uses, a few modals at a time. The most common ones to start with are can (ability), must (obligation), should (advice), and may/might (possibility).

Lock this in: Each modal has a main meaning: can (ability), will (future), should (advice), may/might (possibility), must (obligation). Learn a few at a time with example sentences, don't cram all nine.

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4. The certainty scale (a powerful idea)

Here's one of the most useful things about modals: several of them express how certain you are about something, and they form a scale from very sure to barely possible. This lets you say exactly how confident you are.

From most certain to least certain:

Feel the difference:

• She must be the director. → I'm very confident (from evidence, maybe she's giving orders).
• She might be the director. → I'm just guessing; it's one possibility.

This scale is incredibly useful for expressing yourself precisely: especially in careful or academic writing, where you want to claim exactly as much certainty as your evidence supports (more on this in a later post about "hedging"). Choosing may instead of will is a deliberate signal of how sure you are.

A quick reminder: Modals form a certainty scale: will (~100%, fact) → must (~95%, sure) → should (~70%, expected) → may (~50%) → might/could (~30%) → can't (~0%, impossible). Choose the modal that matches how sure you are. She must be tired (sure) vs. She might be tired (just possible).

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5. Talking about the past: modal + have + past participle

To talk about past possibilities, deductions, or regrets, modals combine with have + past participle. The formula:

Modal + have + past participle

Each combination has a specific meaning:

These are very useful for discussing what did or didn't happen, and for regret or speculation about the past:

• He should have called. → he didn't call, and that was a mistake.
• She must have forgotten. → I'm deducing she forgot.

(Small preview: would have + past participle is exactly the form used in one type of "if" sentence, the Third Conditional, which we'll learn in "If... then...": The Four Conditional Types.)

Say it again: For the past, use modal + have + past participle: must have left (sure she left), should have studied (didn't, a mistake), could have won (didn't), would have helped (hypothetical).

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6. Semi-modals: the modal-like phrases

Some phrases act like modals in meaning but are built differently (they have full verb forms and can change tense). These are called semi-modals. A few common ones:

These are handy because real modals are limited in some ways (for example, you can't easily put two modals together, or change a modal's tense), and semi-modals fill the gaps. Be able to lets you express ability in any tense (will be able to, was able to), where can is more limited.

A useful pair to notice: must and have to both mean obligation, and are often interchangeable (You must submit / You have to submit). A subtle difference: must often feels like the speaker's own rule, while have to often feels like an outside requirement, but for beginners, treating them as similar is fine.

Hold onto this: Semi-modals act like modals but have full verb forms: have to (obligation), need to (necessity), be able to (ability in all tenses), be going to (near future), used to (past habit), ought to (moral obligation).

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7. Where this sits

Modals fit neatly into everything you've learned.

From Reading Time and Voice Off the Page: The Tense Matrix (the verb chain): a modal is just another link in the chain of helping verbs. will be repaired = modal will + Boot be + past participle. When you read a verb chain, a modal at the front tells you the "force" (certainty, obligation, etc.), and the rest of the chain tells you the tense and voice.

From How to Make Questions: The Inversion Algorithm and Saying "No": How to Make Sentences Negative (questions and negatives): modals make these especially easy, because the modal IS the helping verb. So you don't need do, just move the modal for questions (Can she go?) or add not after it for negatives (She cannot go).

And the certainty scale (section 4) previews a key advanced-writing skill called hedging, claiming exactly as much certainty as your evidence allows, which we'll study in the academic-writing post. Modals are the main tool for that.

One more time: A modal is a helping verb in the chain. Questions: just move it (Can she go?). Negatives: add not after it (She cannot go). No do needed.

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8. Have a turn

Try each yourself first, then check the answers.

Exercise A, Choose the modal that fits the meaning

1. You ___ wear a seatbelt. (it's the law, obligation)
2. It ___ rain later. (possibility, about 50%)
3. She ___ speak three languages. (ability)
4. You ___ see a doctor about that. (advice)
5. That ___ be right, the math is wrong. (impossibility)

Exercise B, Fix the structure

1. She can runs fast.
2. They must to leave now.
3. He should studying more.
4. The report can submitted online. (make it a correct passive)

Exercise C, Past modals (modal + have + P.P.)

Choose the form that matches the meaning.

1. She ___ already ___, her coat is gone. (past deduction: must)
2. He ___ ___ harder before the exam. (past advice not taken: should)
3. We ___ ___ you if we had known. (past hypothetical: would)

Exercise D, Certainty scale: rank them

Put these in order from MOST certain to LEAST certain: might, will, may, must

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Answers (try first, then check!)

Exercise A, Choose the modal:

1. must (obligation, the law)
2. may (possibility, ~50%), might is also acceptable
3. can (ability)
4. should (advice)
5. can't / cannot (impossibility)

Exercise B, Fix the structure:

1. She can run fast. (base form, no -s)
2. They must leave now. (base form, no to)
3. He should study more. (base form, no -ing)
4. The report can be submitted online. (modal + be + past participle)

Exercise C, Past modals:

1. She must have already left, her coat is gone.
2. He should have studied harder before the exam.
3. We would have helped you if we had known.

Exercise D, Certainty scale (most → least certain): will (~100%) → must (~95%) → may (~50%) → might (~30%)

How did you do? Exercise B is the structural one (modal + base verb), and Exercise D shows whether you grasp the certainty scale. If you got those, you understand both the form and the power of modals.

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9. The crux of it

1. A modal verb (can, could, will, would, shall, should, may, might, must) goes before a main verb to add meaning: ability, possibility, permission, obligation, advice.

2. The one structural rule: a modal is always followed by the base verb (no -s, -ed, -ing, or to). She can run. For passive: modal + be + past participle.

3. Main meanings: can (ability), will (future), should (advice), may/might (possibility), must (obligation). Learn a few at a time with examples.

4. The certainty scale: will (~100%) → must (~95%) → should (~70%) → may (~50%) → might/could (~30%) → can't (~0%). Choose the modal that matches your confidence.

5. For the past: modal + have + past participle (must have left, should have studied, would have helped).

6. Semi-modals act like modals with full verb forms: have to, need to, be able to, be going to, used to, ought to.

7. In questions and negatives, the modal IS the helping verb, just move it (Can she go?) or add not (She cannot go). No do needed.

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10. One more before you go

You've now gained one of the most expressive tools in English, the modals. You can now express not just what happens, but how able, required, possible, or certain it is. That's a big leap toward sounding natural, precise, and nuanced.

In "If... then...": The Four Conditional Types, we'll use modals (especially will, would, and would have) to build one of English's most useful structures: conditional sentences, the "if... then..." patterns. You'll learn the four standard types, distinguished by their verb forms, so you can talk about real possibilities, unlikely situations, and things that didn't happen in the past.

When you're ready, "If... then...": The Four Conditional Types puts the modals to work. First, build a few sentences with different modals to express different levels of certainty.