Modal verbs are the small helping words that let you talk about ability, permission, obligation, advice and how certain you are. Can, could, may, might, must, shall, should, will and would are the core nine. Alongside them sit the semi-modals ought to, have to and need to, which do the same jobs but behave a little differently. The good news: the core modals all follow the same three grammar rules, so once you have those, you only have to learn the meanings.
The shared rules that make modals easy
Ordinary verbs change form and need do for questions. Modal verbs do not. They share three rules, and these rules apply to every modal in the list.
| Rule | Right | Wrong |
|---|---|---|
| No -s in the third person | She can swim. | She cans swim. |
| Followed by the bare infinitive (no to) | I must go. | I must to go. |
| Questions and negatives without do | Can you help? / You can't go. | Do you can help? |
A few points worth fixing in your memory:
- The verb after a modal is always the base form: go, be, eat, work. No -s, no -ed, no to. He should studies and He should to study are both wrong; it is He should study.
- To make a question, put the modal first: Should I call her? Will they come?
- To make a negative, add not after the modal: I cannot come. You should not worry. The short forms are can't, won't, shouldn't, mustn't, wouldn't and so on.
The semi-modals have to and need to are the exception: they do use do for questions and negatives (Do you have to leave? You don't need to wait), because grammatically they behave more like ordinary verbs.
Ability: can, could, be able to
Can is the everyday word for ability in the present. Could is its past form. Be able to fills the gaps where can and could have no form, such as the future or the perfect.
| Modal | Use | Example |
|---|---|---|
| can | present ability | I can speak three languages. |
| could | past or general ability | She could read at the age of four. |
| be able to | future, perfect, or after another modal | I will be able to drive next year. |
One trap: for a single successful action in the past, English prefers was/were able to or managed to, not could. I could pass the exam sounds wrong for one event; say I was able to pass the exam. (Could is fine for general past ability: I could swim when I was young.)
Permission: can, could, may
All three ask for or give permission. They differ mainly in formality, not meaning.
| Modal | Register | Example |
|---|---|---|
| can | informal, everyday | Can I sit here? |
| could | polite, slightly softer | Could I borrow your pen? |
| may | formal | May I leave early today? |
To give permission, can and may are normal (You can go now. You may begin). To refuse it, use can't or may not: You can't park here. Visitors may not enter.
Obligation and necessity: must, have to
This is where the most useful distinction in the whole topic lives.
| Modal | Source of obligation | Example |
|---|---|---|
| must | the speaker's own authority or strong personal feeling | I must finish this tonight. |
| have to | an external rule or someone else's authority | I have to wear a helmet on site. |
If the pressure comes from you, must fits. If it comes from a law, a boss or a rule, have to fits. In speech the two often overlap, but the source is the real difference.
The negatives are not interchangeable, and this catches almost everyone:
| Negative | Meaning | Example |
|---|---|---|
| mustn't | it is forbidden (prohibition) | You mustn't tell anyone. (Telling is not allowed.) |
| don't have to | there is no obligation (it is optional) | You don't have to tell anyone. (You can if you want.) |
You mustn't come and You don't have to come mean opposite things. The first forbids it; the second says it is up to you.
Advice: should, ought to, had better
These three recommend a course of action, getting stronger as you go down the list.
| Modal | Strength | Example |
|---|---|---|
| should | normal advice | You should see a doctor. |
| ought to | the same as should, slightly more formal | You ought to apologise. |
| had better | a warning; there is a bad result if you don't | You'd better leave now or you'll miss it. |
Should and ought to mean almost the same thing. Had better is stronger and carries a hint of threat or urgency; it always points to a present or future situation, never a general truth. Note its odd shape: it uses had but talks about now, and its negative is had better not (You'd better not be late).
Possibility, probability and deduction
Modals let you say how sure you are about something. This is one of the most useful jobs they do, and it works on a scale of certainty.
| Modal | Certainty | Example |
|---|---|---|
| must be | you are certain it is true | The lights are on - she must be home. |
| can't be | you are certain it is not true | He can't be 40, he looks 25. |
| might / may / could be | it is possible, you are not sure | They might be stuck in traffic. |
Note that the negative of must be (certain something is true) is can't be, not mustn't be. Mustn't is for prohibition, not deduction.
For deductions about the past, use modal + have + past participle:
- She must have left. (I am sure she left.)
- He can't have known. (I am sure he didn't know.)
- They might have missed the train. (It is possible they did.)
Common mistakes
Ranked by how often they actually appear:
- Adding -s in the third person. She cans drive, he musts go. Modals never take -s. It is She can drive, he must go.
- Putting to after a modal. I must to go, you should to rest. The verb after a core modal is the bare infinitive: I must go, you should rest. (Ought to, have to and need to keep their to, because the to is part of the semi-modal itself.)
- Confusing mustn't with don't have to. You mustn't pay (it is forbidden to pay) when you mean You don't have to pay (paying is optional). They are opposites; choose carefully.
- Using do with a modal. Do you can come? Does she must stay? Modals make their own questions: Can you come? Must she stay?
- Using can for the past. Yesterday I can't sleep. For the past you need could: Yesterday I couldn't sleep.
Practice
Choose the correct modal. Answers are below.
- She
___(can / cans) play the violin beautifully. - You
___(mustn't / don't have to) touch that wire - it's dangerous. - I'm not sure, but they
___(must / might) be at home. ___you help me with this box, please?- When I was a child I
___(can / could) climb any tree in the garden.
Answers: 1. can (modals never take -s) 2. mustn't (it is forbidden, a real danger) 3. might (you are not certain) 4. Can / Could (Could is more polite) 5. could (general ability in the past)