Modals — Quick Reference
The Rule in 10 Seconds
Section titled “The Rule in 10 Seconds”Modal verbs are helper verbs that change the mood of a sentence — ability, permission, obligation, possibility. They follow one golden rule: modal + base verb, no “to”, no “-s”.
Modal Functions at a Glance
Section titled “Modal Functions at a Glance”| Function | Modal | Example |
|---|---|---|
| Ability | can, could | She can swim. |
| Permission | can, could, may | You may leave. |
| Obligation | must, have to | You must stop. |
| No obligation | don’t have to | You don’t have to wait. |
| Prohibition | must not | You mustn’t cheat. |
| Advice | should, ought to | You should rest. |
| Possibility | might, may, could | It might rain. |
| Deduction (sure) | must | She must be tired. |
| Deduction (impossible) | can’t | He can’t be 80. |
Modal Formation
Subject + modal + base verb
→ She can swim. NOT She can to swim / She can swims
The Big Trap: mustn’t ≠ don’t have to
Section titled “The Big Trap: mustn’t ≠ don’t have to”| Meaning | Example | |
|---|---|---|
| mustn’t | Forbidden 🚫 | You mustn’t steal. (Don’t do this!) |
| don’t have to | Optional — your choice | You don’t have to come. (Up to you.) |
Past Modals (Deduction & Regret)
Section titled “Past Modals (Deduction & Regret)”| Modal + have + V3 | Meaning | Example |
|---|---|---|
| must have | I’m sure it happened | She must have left. (Her car is gone.) |
| can’t have | I’m sure it didn’t happen | He can’t have finished already. |
| could have | It was possible but didn’t happen | You could have told me! |
| should have | It was the right thing, but you didn’t | I should have studied more. |
| might have | Maybe it happened | They might have forgotten. |
| needn’t have | You did it, but it wasn’t necessary | You needn’t have worried. |
Modal Perfect
Subject + modal + have + past participle (V3) → She must have left. / I should have known.
Common Mistakes
Section titled “Common Mistakes”He can to swim.
He can swim.
Modals never take 'to' before the verb. Modal + base verb, always.
She musts go.
She must go.
Modals don't add -s for third person. No conjugation, ever.
You mustn't pay — it's free.
You don't have to pay — it's free.
Mustn't = forbidden. Don't have to = not necessary. Free parking isn't forbidden.
I should study harder last year.
I should have studied harder last year.
Past regret needs modal + have + V3, not modal + base verb.
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