Modal verbs add the speaker’s attitude to the main verb: ability (can), permission (may), obligation (must), advice (should), possibility (might). The same sentence changes its meaning completely with a different modal: He must be at home (I’m sure he is), He might be at home (perhaps he is), He can’t be at home (it’s impossible that he is). Exams test choosing a modal by meaning, the difference between mustn’t and don’t have to, and deductions about the past (must have done).
§1. General rules for modal verbs
Modals behave differently from ordinary verbs:
They do not change for person and never take -s: She can swim. (✗ She cans)
They are followed by the base form without to: You must go. (✗ must to go)
Questions are formed without do, by inversion: Can you help me? (✗ Do you can)
Negatives use not: cannot/can’t, mustn’t, shouldn’t, might not.
They have no infinitive or -ing form, so other tenses use substitutes: can → be able to, must → have to.
modal + V (no to) · Question: Modal + subject + V? · Negative: modal + not + V
Core modals: can, could, may, might, must, shall, should, will, would. Semi-modals (which behave like ordinary verbs): have to, need to, be able to, ought to.
§2. Ability and permission
Ability:
can — present ability: I can speak three languages.
could — general ability in the past: I could read when I was four.
was/were able to — managed to in one specific past situation: The fire was big, but everyone was able to escape. (✗ could escape in this meaning). In the negative, couldn’t is also fine: I couldn’t open the door.
be able to in other tenses: I will be able to come.I’ve never been able to swim.
Permission:
can — neutral: Can I use your phone?
could — polite: Could I ask a question?
may — formal: May I come in?
could is not used in the reply: Yes, you can. / Yes, you may.
could — general past ability · was able to — a one-off success
Worked example
The exam was difficult, but I ___ (solve) all the problems.
The sentence is about one specific past situation (the exam).
The meaning is that I succeeded despite the difficulty.
A success in a specific situation needs was able to, not could.
Answer: I was able to solve all the problems.
§3. Obligation, prohibition and lack of necessity
must — the speaker’s own sense of obligation, or a written rule: I must call my mother.Passengers must wear seat belts.
have to — external obligation (rules, circumstances, other people): I have to wear a uniform at work.
must has no past form; in the past use had to: Yesterday I had to work late. In the future: will have to.
mustn’t — prohibition: You mustn’t smoke here.
don’t have to / needn’t — no necessity (you may do it or not): You don’t have to come — it’s optional.
mustn’t = it is forbidden · don’t have to = it is not necessary
These two negatives mean completely different things, and exams test them often.
Worked example
Tomorrow is Sunday, so I ___ get up early.
Sunday means there is no work or school.
Getting up early is not forbidden; it is simply unnecessary.
No necessity → don’t have to.
Answer: Tomorrow is Sunday, so I don’t have to get up early.
§4. Advice, offers and criticism
should / ought to — advice, the right thing to do: You should see a doctor.You ought to apologise.
had better + V — strong advice with a warning of bad consequences: You’d better hurry or you’ll miss the bus. Negative: You’d better not be late.
Shall I…? — offering help: Shall I open the window?; Shall we…? — suggesting doing something together: Shall we go?
Could you…? / Would you…? — polite requests: Could you help me?
Regret or criticism about the past: should have + V3 — it was the right thing, but it did not happen: You should have told me.I shouldn’t have eaten so much.
§5. Possibility and deduction
Modals also express conclusions based on evidence. Learn the degrees of certainty:
must — almost certain (positive deduction) · may/might/could — possible · can’t — impossible (negative deduction)
About the present (modal + V):
He’s been working all day — he must be tired.
She might be at home.
That can’t be true!
About something in progress now: modal + be + V-ing: She isn’t answering — she must be driving.
About the past (modal + have + V3):
The street is wet — it must have rained.
He may have missed the train.
She can’t have seen me — she didn’t say hello.
Negative deductions use can’t, not mustn’t: ✗ He mustn’t be at home → ✓ He can’t be at home.
Worked example
Anvar isn’t answering. He ___ (be) asleep — it’s 2 a.m.
There is evidence: it is two in the morning and he is not answering.
This is a strong, logical positive conclusion.
A strong positive deduction → must + V.
Answer: He must be asleep.
Worked example
I can’t find my keys. I ___ (leave) them at school.
The conclusion is about a past action (leaving the keys at some point).
The speaker is not sure, so it is a possibility.
Possibility about the past: may/might + have + V3.
The V3 of leave is left.
Answer: I might have left them at school.
§6. A routine for choosing a modal
What is the meaning: ability, permission, obligation, advice or deduction?
What is the time? For the past: could/was able to, had to, should have, must have.
If it is negative, check the meaning: prohibition (mustn’t) or no necessity (don’t have to); a negative deduction is can’t.
For a deduction, judge the certainty: sure → must, possible → might, impossible → can’t.
Check that there is no to and no -s after the modal.
Check yourself
You ___ park here — it’s a fire exit.
When I was young, I ___ run very fast.
It’s a free museum. You ___ pay.
The lights are off. They ___ be at home.
I ___ wear glasses when I drive — it’s the law.
I failed the test. I ___ have studied more.
Yesterday we ___ (must) stay at school until six.
The ground is white. It ___ have snowed at night.
Common mistakes
to after a modal: ✗ I must to go → ✓ I must go (but ought to and have to do take to).
Adding -s: ✗ She cans → ✓ She can.
Confusing mustn’t with don’t have to.
Using must for the past: ✗ Yesterday I must work → ✓ I had to work.
mustn’t in a negative deduction: ✗ He mustn’t be serious → ✓ He can’t be serious.
can for a guess about a specific situation: ✗ He can be at home now → ✓ He may/might be at home now. ( is only for general possibility: )