Modal verbs are helpers that change the meaning of a sentence: can for ability, must for obligation, should for advice, may for permission and possibility. There are only nine of them, and they share the same unusual grammar.
Once you learn the shared rules, the only work left is choosing the right modal for the job. The choice is about strength and politeness: how sure, how required, how polite?
This page groups the modals by job, shows the two traps that catch almost every learner (the extra to, and the missing difference between must not and do not have to), and drills 20 sentences.
The rule
What modal verbs share
- The modals are can, could, may, might, must, shall, should, will and would. Ought to works like a modal.
- They never take -s: she can swim, not she cans swim.
- They are followed by the base verb without to: you must go, not you must to go.
- They make questions and negatives without do: Can you help? She cannot come. You must not smoke here.
- Two modals cannot meet: use be able to, have to or be allowed to instead. "I will be able to come", not "I will can come".
Ability: can and could
- Can is ability in the present: "I can swim."
- Could is general ability in the past: "She could read music when she was six."
- For one successful action in the past, use was able to or managed to: "The fire was fierce, but we were able to escape."
- For the future and perfect tenses, use be able to: "I will be able to help tomorrow", "I have been able to finish."
Permission and requests: can, could, may
- Can is normal and informal for permission: "Can I borrow your pen?"
- Could is more polite: "Could you close the window, please?"
- May is formal: "May I come in?" It is the right choice with strangers, officials and in writing.
- For prohibition, use cannot or must not: "You cannot park here."
Obligation: must, have to and the two negatives
- Must shows a strong obligation from the speaker or from a rule: "You must wear a helmet."
- Have to shows an obligation from outside: "I have to work on Saturday because my boss asked."
- In practice the two often mean the same thing. The past of both is had to: "I had to leave early."
- Must not means it is forbidden: "You must not touch that switch."
- Do not have to means there is no obligation: "You do not have to come, but you are welcome." These two are opposites.
Advice and possibility: should, may, might, could
- Should and ought to give advice: "You should sleep more."
- May, might and could show possibility. Might is the most tentative: "It may rain", "It might rain", "It could rain".
- Degrees of certainty: must be (almost sure), may be or might be (possible), cannot be (impossible).
- "He must be tired" means I am sure he is tired. "He cannot be tired" means I am sure he is not.
- For past deduction, use modal plus have plus participle: "She must have missed the bus."
Examples
- I can speak three languages.Present ability.
- My grandmother could ride a horse.General past ability.
- Could you pass me the salt, please?A polite request.
- May I ask a question?Formal permission.
- You must wear a seatbelt in the car.A rule and a strong obligation.
- I have to pay the rent on the first of the month.An obligation from outside, an arrangement.
- You must not use your phone in the exam.Prohibition.
- You do not have to bring anything to the party.No obligation. It is not forbidden, just not necessary.
- You should drink more water.Advice.
- She must have left already. Her coat is gone.Past deduction: I am sure it happened.
Common mistakes
- Wrong: She cans swim very well. Right: She can swim very well.
- Modal verbs never take -s, whatever the subject.
- Wrong: You must to finish your homework. Right: You must finish your homework.
- A modal is followed by the base verb. The "to" belongs only to have to and ought to.
- Wrong: Do you can help me? Right: Can you help me?
- Modal verbs make questions by moving to the front. They never use do.
- Wrong: You must not come to work tomorrow; it is a holiday. Right: You do not have to come to work tomorrow; it is a holiday.
- Must not means forbidden. No obligation is do not have to. The sentence means the office is closed, not that entry is a crime.
- Wrong: I should to study tonight. Right: I should study tonight.
- Should is a modal, so the verb after it takes no to.
Practice drill
Answer all 20 questions. Each answer is marked at once, and the rule is explained under the question.
She ___ play the violin when she was eight.
Show answer and explanation
Answer: A. could General ability in the past uses could.
___ you help me carry these boxes?
Show answer and explanation
Answer: B. Can Modal verbs make questions without do: Can you help?
You ___ wear a helmet on a building site. It is the law.
Show answer and explanation
Answer: C. must A rule and a strong obligation take must.
I ___ go to the dentist yesterday because my tooth hurt.
Show answer and explanation
Answer: D. had to The past of must and have to is had to.
You ___ bring a towel. The hotel provides them.
Show answer and explanation
Answer: A. do not have to There is no obligation, so use do not have to. Must not would mean it is forbidden.
That ___ be John at the door. He is in Spain this week.
Show answer and explanation
Answer: B. cannot Cannot shows that something is impossible. Must would mean I am almost sure it is him.
You ___ drink more water if you get headaches.
Show answer and explanation
Answer: C. should Should is used to give advice.
___ I open the window? It is very warm in here.
Show answer and explanation
Answer: D. May May is the formal, polite way to ask for permission. Can is also correct but less formal.
She ___ be at home. I saw her car in the driveway.
Show answer and explanation
Answer: A. must Must shows strong certainty that something is true.
I ___ swim when I was five, but I learnt quickly.
Show answer and explanation
Answer: B. could not Could not is lack of ability in the past. Must not and should not are about obligation, not ability.
The train has gone. We ___ have missed it.
Show answer and explanation
Answer: C. must Past deduction uses must have plus the past participle: we must have missed it.
Students ___ use dictionaries during the exam. It is not allowed.
Show answer and explanation
Answer: D. must not Not allowed means prohibition, so use must not.
___ you speak more slowly, please?
Show answer and explanation
Answer: A. Could Could makes a polite request. Should would ask about obligation.
I ___ go to the meeting tomorrow. I have no choice.
Show answer and explanation
Answer: B. have to No choice means an outside obligation: have to.
You look tired. You ___ go to bed earlier.
Show answer and explanation
Answer: C. should Should gives friendly advice.
He ___ be at the office. It is Sunday, and he never works Sundays.
Show answer and explanation
Answer: D. cannot Cannot expresses certainty that something is impossible. The facts make his presence at the office impossible.
Passengers ___ smoke anywhere on the plane.
Show answer and explanation
Answer: A. must not A rule that forbids something takes must not. Should not is advice, and do not have to means it is not required.
We ___ finish the project by Friday, or the client will be angry.
Show answer and explanation
Answer: B. must A strong obligation with a consequence takes must.
She ___ have taken the wrong train. She is not answering her phone.
Show answer and explanation
Answer: C. must Must have plus the past participle shows a confident guess about the past.
When I was a child, I ___ climb that tree easily.
Show answer and explanation
Answer: D. could Could describes general ability in the past.
Questions learners ask
What is the difference between "must" and "have to"?
Both express obligation. Must usually comes from the speaker or from a written rule: "You must stop at a red light." Have to usually comes from an outside situation: "I have to take my car to the garage." In everyday speech the difference is small, and the past form of both is had to.
Should or could for advice?
Should is a recommendation and sounds like the best option: "You should see a doctor." Could offers an option without pressure: "You could try turning it off and on again." Use should for advice, and could for suggestions.
Can I use two modals in one sentence?
No. English does not allow "will can" or "might must". Replace the second modal with a full phrase: "I will be able to help", "You might have to wait", "She may be allowed to leave early".
More grammar topics
Can vs could
Can for now, could for the past and for polite requests, plus the one past situation that refuses could.
Gerund vs infinitive
Verb plus -ing or verb plus to: the pattern lists, and the verbs where meaning flips.
Past continuous
Was or were plus -ing: the action in progress, the interruption, and the background of a story.
Take the free A1 to C2 level test to find your starting point.