Answer in brief
Modal verbs are helper verbs — can, could, may, might, must, shall, should, will and would — that add ability, permission, obligation, advice or certainty to a main verb. They take no -s, no do in questions and negatives, and no to before the next verb. The pair to learn first is mustn’t (it is forbidden) against don’t have to (it is not necessary).
Which verbs are modal and the three rules they all share
Modal verbs are the helper verbs of English — can, could, may, might, must, shall, should, will and would — that add ability, permission, obligation, advice or a degree of certainty to a main verb. They share three rules: no -s after he, she or it (“she can”), no do in questions and negatives (“Can you swim?”, “I can’t”), and a bare infinitive without to (“You must go”). The British Council’s LearnEnglish grammar lists exactly these nine as the modal verbs.
Wikipedia’s article on English modal auxiliary verbs explains the rules: modals lack the third-person -(e)s, take only a bare infinitive and have no untensed forms, so there is no to can or will can. In questions they swap places with the subject (“Must I go?”), and Wikipedia’s entry on do-support notes that do is not used when an auxiliary is already present. Around the core, Purdue University’s Online Writing Lab lists semi-modals such as have to, ought to and be able to. They do similar work but are followed by to.
Talking about ability with can, could and be able to
For skills, English starts with can. The British Council’s page on can and could uses can and can’t for general abilities (“She can speak several languages”) and for ability at a given moment (“I can see you”). Wikipedia adds that could is the only modal regularly used as an ordinary past tense, and only when it refers to ability: “I couldn’t see you.”
Trouble starts when you need a form that can lacks. Modals have no infinitive and no future, so “I will can drive” is impossible. Wikipedia’s article says that can and could, when they express ability, can be replaced by be able to with the appropriate form of be. Hence “I will be able to drive”, “I have been able to save a little” and “I would like to be able to swim”.
Permission and polite requests with can, could and may
Here the modal changes the tone, not the meaning. The British Council’s lesson on permission and obligation sets three in a line: can asks for and gives permission; could only asks and is more formal and polite than can; may is the most formal way to ask and to give. “Can I sit here?” suits a café, “Could I borrow your pen for a moment, please?” a colleague, and “May I see your passport, please?” an official.
The answer to “Could I open the window?” is “Yes, you can”, because could does not give permission. When you ask someone else to act, the British Council calls “Could you…?” the polite way and notes that can is less polite. Wikipedia’s article finds “Might I use your phone?” more hesitant or polite still. Rules that forbid something use can’t: “You can’t take photos in the museum.”
Must or have to, mustn’t or don’t have to, and should
Must and have to both say that something is necessary. The British Council sees a slight difference: have to shows an obligation from outside the speaker, while must expresses a strong obligation, often from the speaker or the author of a rule (“Seat belts must be worn by all passengers”). Wikipedia contrasts “I must go into town”, your own urgency, with “I have to go into town”, a duty set by others or by circumstances. For the past, the British Council says, only had to works.
In the negative the two part completely. Mustn’t is a prohibition: in the British Council’s “You mustn’t make any noise in the library”, silence is required. Don’t have to means no obligation: you can if you want to, but it is not compulsory (“You don’t have to come with me”). Wikipedia explains that the negation of must falls on the action, so to say that something is merely unnecessary, English uses the negative of have to, or needn’t.
Advice sits one step lower. The British Council lists should and shouldn’t for suggestions and advice, and Wikipedia describes should as lacking the strong obligatory force of must or have to. Ought has similar meanings but, unlike the core modals, takes an infinitive with to: “You ought to call her”. Have to, a semi-modal in Wikipedia’s list, changes with the person and needs do: “Does she have to work on Saturday?”
Possibility and deduction with may, might, must and can’t
Modals also measure how sure you are. The British Council’s page on probability says may, might and could mark something as possible but not certain: “They might be at home”, “If we don’t hurry, we could be late”. Should sits higher: for the British Council, it suggests something is true when we have reasons, as in “Ask Miranda. She should know.”
At the ends of the scale stand must and can’t. Must, the British Council explains, shows we are sure and have reasons: “You haven’t eaten all day. You must be hungry.” The lights are on, so “She must be at home.” The opposite of this must is not mustn’t but can’t, which marks something as impossible: “That can’t be Peter’s coat. It’s too small.” Say “He mustn’t be at home” and a guess turns into a ban.
Modal perfects: should have, must have and might have done
For the past, modals keep their own form and add have plus a past participle; Wikipedia’s article gives this as the way modals take past reference. Must have done then refers only to certainty, never to a past duty, which is had to. Should have done usually expresses something that would have been expected in the past. The British Council’s example “You should have helped her when she asked” carries the regret or reproach that the help never came.
For guesses, the British Council’s lesson on deductions about the past gives a set. Must have means you feel sure: “The thief must have had a key.” Might have and may have mark a possibility, may have being the more formal. Can’t have and couldn’t have rule it out: “She can’t have driven there. Her car keys are still here.”
Typical mistakes with modals and how to correct them
Most errors break a form rule. “I must to go” becomes “I must go”: a bare infinitive follows. “He cans swim” becomes “He can swim”: there is no third-person -s. “Do you can help me?” becomes “Can you help me?”: the modal moves in front of the subject. “I will can come” becomes “I will be able to come”. And Purdue’s Online Writing Lab warns against could of and would of, which copy how could have sounds in speech.
Meaning errors are rarer but cost more. “You mustn’t bring anything”, said to guests who may come empty-handed, sounds like a ban; the right sentence is “You don’t have to bring anything”. A guess needs “He can’t be at home”, not mustn’t. The reply to “Could I…?” is “Yes, you can”. A finished duty needs had to: “Yesterday I had to stay late.” Ask each time whether you mean a rule, a permission or a conclusion.
Your first language adds its own traps. Wiktionary glosses the Russian должен as must, have to, should or supposed to, so the strength has to be chosen every time. In Wiktionary’s entry the Spanish deber means must or have to, its conditional gives should, and the Royal Spanish Academy prefers deber de for probability. The French devoir covers must, have to and, in the conditional, should. Wiktionary translates “Il ne faut pas que tu dises ça” both as “You don’t have to say that” and as “You mustn’t say that”, the very line English will not blur.
Practising modals alone this week, and what the 113-day course adds
Practise alone first. Take one meaning a day — ability, permission, obligation, advice, possibility, deduction, the past — and write five true sentences about your life for each. Read them aloud, then rewrite each with a contrasting modal, must into don’t have to or might into can’t. Keep a list of your own errors. What is hard to supply alone is a steady order of topics and someone to point out the mistake you do not hear.
VITON13 SCHOOL builds that order into its online course English in 113 Days: 113 levels, one a day, with explanations in Russian, French, Spanish, German or Portuguese — the learner picks the language in the classroom. Modals return across it: can and can’t at level 17, should and shouldn’t at 35, have to and don’t have to at 36, must, might and can’t for deductions at 58, modal perfects at 80, and the passive with modals, such as should have been done, at 87.
Each level brings 13 new words spoken aloud with transcription and an example, a three-minute grammar point with the mistake speakers of your language usually make, a short text, dictation, your own writing with a model answer and a mini-game. Learnt words join your memory cards, where the FSRS algorithm of the open-source ts-fsrs library brings each back as it starts to be forgotten. The levels follow the Council of Europe’s CEFR scale from A1 to B2, with a bridge towards C1. The first three levels open without payment or sign-in.
Practical checklist
- Write five true sentences about your week with can, could and be able to.
- List three rules from your workplace or school and sort them into mustn’t and don’t have to.
- Say one deduction aloud each day with must be, then its opposite with can’t be.
- Rewrite one past regret with should have and one guess about yesterday with might have.
- Check every modal you write for a stray to, an extra -s or an unneeded do.
Questions and answers
Is could simply the past tense of can?
Only in part. Wikipedia points out that could is the one modal regularly used as an ordinary past tense, and then only for ability: I could swim at six. Elsewhere it is not past at all: the British Council lists could for polite requests, for asking permission and for things that are possible but not certain.
When should I use should instead of must?
Use should for advice and expectations, must for a strong obligation or a firm conclusion. The British Council gives should and shouldn’t for suggestions and advice, and Wikipedia describes should as lacking the obligatory force of must. You should rest sounds like a friend’s advice; you must rest sounds like a doctor’s order.
Which is right for a rule at work, must or have to?
Both are understood. The British Council links have to with obligations that come from outside the speaker and must with obligations set by the speaker or by the author of a rule, which is why signs and regulations often use must. For a duty in the past, only had to works.
Why is there no to after can, must and should?
Because a modal takes a bare infinitive: can swim, must leave, should call. Wikipedia’s article says a modal can, in principle, take only a bare infinitival clause. The exceptions are the semi-modals: ought to, have to and be able to keep their to, which is why they are best learnt as whole phrases.
How can I practise English modals without boring drills?
Tie each modal to your own life. Write the rules of your home with mustn’t and don’t have to, plans with will be able to, guesses about a colleague’s day with might and must, and last week’s regrets with should have. Say the sentences aloud and come back to them a few days later, so the examples stay yours rather than a textbook’s.