How to Use Modal Verbs

What are modal verbs?

Modal verbs are auxiliary (helping) verbs that express attitude, possibility, obligation, or ability. They always come before the base form of the main verb, and they do not change form — no -s in the third person, no -ing, no infinitive with to.

Rule	Correct	Incorrect
No -s in third person	She can swim.	She cans swim.
No infinitive with to	You should call him.	You should to call him.
Followed by base verb	They must leave now.	They must leaving now.
No auxiliary for questions/negatives	Can you help?	Do you can help?

Ability — can, could, be able to

Use can for present ability and could for past ability or polite requests. For ability in other tenses, use be able to.

Ability

She can speak three languages. (present ability)

He could run very fast when he was young. (past ability)

I haven't been able to contact her. (present perfect — no 'have could')

Polite requests

Can you open the window? (informal)

Could you open the window? (more polite)

Tip: Use could rather than can in formal writing and polite speech. Could also sounds more tentative and less demanding.

Possibility — may, might, could

These three modals all express possibility, but with different degrees of certainty. May suggests a stronger possibility than might.

mustalmost certain

shouldprobable

maypossible (~50%)

mightless likely

couldremotely possible

Examples

It may rain this afternoon. (around 50% likely)

It might rain — but I doubt it. (less certain)

Take an umbrella. It could be useful. (remote possibility)

Possibility in the past

To express possibility in the past, use may have, might have, or could have + past participle.

She might have missed the train.

He may have already left.

They could have taken a different route.

Obligation and necessity — must, have to, should

These modals express how necessary or important something is, but with key differences.

Modal	Meaning	Example
must	Strong obligation (speaker's authority); logical deduction	You must wear a seatbelt. / She must be exhausted.
have to	External obligation (rules, laws, circumstances)	I have to submit this by Friday. (my boss said so)
should	Advice or moral expectation	You should see a doctor about that.
ought to	Similar to should; slightly more formal	You ought to apologise.
need not	No obligation — it is unnecessary	You needn't bring anything.
must not	Strong prohibition — do not do this	You must not smoke in here.

Key contrast: Don't have to means it is not necessary. Must not means it is forbidden. These are very different: You don't have to eat it (no obligation) vs. You must not eat it (it's prohibited).

Permission — can, may, could

All three can be used to ask for or give permission, but the level of formality varies.

Modal	Context	Example
can	Informal permission	Can I borrow your pen?
could	Polite request for permission	Could I leave early today?
may	Formal permission	May I ask a question?

Tip: In formal or professional situations, may I is the most appropriate choice. In everyday speech, can I is perfectly acceptable.

Future and hypothetical — will, would, shall

Will

Use will for predictions, spontaneous decisions, promises, and offers.

It will be warm tomorrow. (prediction)

I'll get the door — don't worry. (spontaneous offer)

I promise I'll be there. (promise)

Would

Use would for hypothetical situations (second and third conditionals), polite requests, and past habits.

If I had more time, I would travel more. (hypothetical)

Would you like a coffee? (polite offer)

When I was young, we would visit my grandparents every Sunday. (past habit)

Shall

Shall is mainly used in British English, in offers and suggestions with I and we.

Shall I open the window? (offer)

Shall we begin? (suggestion)

Common mistake: Never use would in the if-clause of a conditional. ✗ If I would have time → ✓ If I had time.

Quick reference — all modals at a glance

Modal	Key uses	Example
can	Ability, informal permission, requests	Can you swim?
could	Past ability, polite requests, possibility	Could you help me?
may	Formal permission, possibility (~50%)	It may snow tonight.
might	Weaker possibility, tentative suggestions	He might be late.
must	Strong obligation, logical deduction	You must try this.
must not	Prohibition	You must not enter.
should	Advice, expectation, probability	You should rest.
will	Future facts, predictions, promises	I will call you later.
would	Hypothetical, polite requests, past habits	Would you mind waiting?
shall	Offers and suggestions (British English)	Shall we go?