Modal Verbs: Ability, Permission, Obligation & Probability

1.

MODAL VERBS
Ability, Permission, Obligation & Probability
English Grammar — Grade 11
can • could • may • might • must • should • will • would • ought to

2.

What Are Modal Verbs?
Modals are helping verbs that add meaning to another verb — showing ability, permission, obligation, or probability. They never change form
and are always followed by a base verb.
can
could
may
might
must
shall
should
will
would
ought to
Three rules that never change
No -s ending
She can swim. (never “cans”)
Followed by base verb
You must go. (never “to go”)
No do/does needed
Can you help? (never “Do you can”)

3.

Ability & Possibility
can, could, be able to — what someone is (or was) capable of doing.
can
could
be able to
General ability, present tense
Past or general ability
All tenses, especially future
She can speak three languages.
I could swim before I could walk.
He'll be able to drive next year.

4.

Permission
Asking for or giving permission — the tone shifts with the modal you choose.
Informal
Formal
can
may
“Can I use your phone?” — everyday, casual speech.
“May I ask you a question?” — polite, respectful requests.
could
may not / must not
“Could I leave early today?” — a little more polite.
“You must not park here.” — formally denying permission.

5.

Obligation & Necessity
must vs. have to — both express necessity, but the source of the rule is different.
must
have to
Personal obligation — the speaker's own feeling that something is
necessary.
External obligation — a rule or requirement that comes from
somewhere else.
“I must call my mom — I promised her.”
“I have to wear a uniform — it's the school rule.”
must not = it is prohibited
don't have to = it's not necessary
“You must not smoke here.” (it's forbidden)
“You don't have to come — it's optional.” (there's a choice)

6.

Advice & Suggestion
should, ought to, had better — recommending the right course of action.
should
The most common way to give advice — gentle
and general.
“You should see a doctor.”
ought to
Very close to should, slightly more formal.
“We ought to leave now.”
had better
Stronger — implies a real consequence if
ignored.
“You'd better hurry, or you'll miss the bus.”

7.

Probability & Certainty
The modal you pick signals how sure you are about something.
must
should
may / might
could
can't
Almost certain
Likely / expected
Possible, not sure
Possible, weaker guess
Almost certainly not
“The lights are off — they must be
asleep.”
“She should be home by now.”
“It might rain later.”
“That could be Tom's car.”
“He can't be at work — it's a
holiday.”

8.

Modals in the Past
Modal + have + past participle — for looking back with regret, guessing, or blame.
should have + V3
Regret about a past action (or lack of one)
“I should have studied harder.”
could have + V3
Something was possible but didn't happen
“She could have won, but she gave up.”
must have + V3
A confident guess about the past
“The ground is wet — it must have rained.”
might have + V3
An uncertain guess about the past
“He might have missed the bus.”

9.

Common Mistakes
Watch for these mix-ups in your own writing.
Adding ‘to’ after a modal
Using ‘-s’ with modals
✗ She can to swim.
✗ He musts leave now.
✓ She can swim.
✓ He must leave now.
Confusing must not / don't have to
Wrong form after modal + have
✗ You must not come (meaning optional).
✗ I should have went earlier.
✓ You don't have to come (it's optional).
✓ I should have gone earlier.

10.

Quick Practice
1. You ______ wear a seatbelt — it's the law. (obligation)
2. She ______ speak French when she was five. (past ability)
3. The lights are on — they ______ be home. (confident guess)
4. ______ I open the window? (polite permission)
Ability
Permission
Obligation
Advice
Probability
Past Modals