Key rules for all modals
| Rule | Correct | Wrong |
|---|---|---|
| No -s in third person | She can swim. | ✗ She cans swim. |
| Followed by infinitive (no to) | You must go. | ✗ You must to go. |
| No -ing form / no past tense | I must leave. → use had to for past | ✗ I musted leave. |
| Negative: modal + not | You cannot / can't park here. | ✗ You not can park here. |
| Question: inversion only | Can you help me? | ✗ Do you can help me? |
Functions at a glance
| Function | Modals | Example |
|---|---|---|
| Ability | can / can't · could / couldn't · was able to | She can speak four languages. |
| Permission | can · may (formal) · could (polite) | May I leave early today? |
| Obligation | must · have to · should · ought to | You must wear a seatbelt. |
| Prohibition | mustn't · can't · may not | You mustn't smoke here. |
| No obligation | don't have to · needn't | You don't have to pay — it's free. |
| Advice | should · ought to · had better | You should see a doctor. |
| Possibility / probability | might · may · could · must · can't | It might rain later. |
| Request | can · could (polite) · will · would (polite) | Could you open the window? |
| Offer / suggestion | shall · can · could · would | Shall I carry that for you? |
had better:
Stronger and more urgent than should. Implies a negative consequence if you don't follow the advice: "You had better call him now — he's furious." Often shortened to you'd better.
Exercises
Choose the correct modal for each situation.
1. You want to ask permission politely in a formal situation.
2. Something is forbidden — it is not allowed.
3. You want to make a polite request.
4. You want to offer help: "___ I carry your bags?"