🎯 Modal Verbs: Can, Must, Should & More
English modal verbs for ability, obligation, permission, and probability. Free English grammar guide for learners: clear rules, examples, and patterns.
The Modal System
Modal verbs are special auxiliary verbs that express ability, permission, obligation, or probability. They follow different grammar rules than normal verbs: no -s in third person, no "do" in questions, and they're followed by the bare infinitive (without "to").
| Modal | Main Meaning | Example |
|---|---|---|
| can | ability, permission | I can swim. Can I sit here? |
| could | past ability, polite request | I could swim when I was 5. Could you help me? |
| may | permission (formal), possibility | May I come in? It may rain later. |
| might | possibility (less certain) | She might be late. |
| must | strong obligation, logical certainty | You must wear a seatbelt. He must be tired. |
| shall | suggestion, offer (BrE) | Shall we go? Shall I open the window? |
| should | advice, expectation | You should see a doctor. She should be here soon. |
| will | future, willingness | I will help you. It will rain tomorrow. |
| would | conditional, polite request | I would go if I could. Would you like tea? |
Ability & Permission
For talking about what someone is able to do or is allowed to do:
| Expression | Use | Example |
|---|---|---|
| can | Present ability | She can play the piano. |
| can | Informal permission | You can use my phone. |
| could | Past ability | I could read when I was four. |
| could | Polite request | Could I borrow your pen? |
| be able to | All tenses (replaces can) | I will be able to come tomorrow. |
| may | Formal permission | May I leave early today? |
| be allowed to | All tenses (replaces may) | We weren't allowed to enter. |
Obligation & Advice
This is where the most dangerous confusion happens — mixing up "must not" and "don't have to" can completely reverse your meaning:
| Expression | Meaning | Strength | Example |
|---|---|---|---|
| must | It's necessary (internal feeling or rule) | Strong obligation | I must study harder. |
| have to | It's necessary (external rule) | Strong obligation | I have to wear a uniform. |
| must not | It's FORBIDDEN — don't do it! | Prohibition | You must not park here. |
| don't have to | It's NOT necessary — your choice | No obligation | You don't have to come. |
| should | It's a good idea | Advice | You should eat more vegetables. |
| ought to | It's the right thing (slightly formal) | Advice | We ought to help them. |
| had better | Strong advice (with consequences) | Warning | You'd better hurry or you'll miss the bus. |
Probability & Deduction
Modal verbs can express how certain you are about something. The scale ranges from almost certain to very unlikely:
| Modal | Certainty Level | Example | Meaning |
|---|---|---|---|
| must | ~95% certain | He must be at home. | I'm almost sure he is. |
| will | ~90% certain | That will be John at the door. | I expect/predict it is. |
| should | ~75% certain | She should be there by now. | I expect so (based on normal timing). |
| may | ~50% certain | It may rain tomorrow. | It's possible. |
| might/could | ~30% certain | She might be sick. | It's possible but I'm not sure. |
| can't/couldn't | ~95% certain NOT | That can't be true! | I'm almost sure it isn't. |
Common Modal Verb Mistakes
These are the errors that German and Russian speakers make most often with modals:
| Mistake | Correct | Why |
|---|---|---|
| "I must to go." | "I must go." | No "to" after modals — bare infinitive only. |
| "He cans swim." | "He can swim." | Modals never take -s, even with he/she/it. |
| "Do you can help me?" | "Can you help me?" | No "do" with modals — just invert. |
| "I mustn't go" (meaning: not necessary) | "I don't have to go." | German "muss nicht" = "don't have to," NOT "mustn't." |
| "I didn't must." | "I didn't have to." | Use "had to/have to" for past forms. |
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