Modal Verbs

How Strong Is It?

Same modal idea, different jobs — this scale is just for the obligation/necessity line. Possibility, prediction and deduction each get their own logic in the cards below.

Can / Could

Structure: can / could + base verb

I can swim. / Could you help me?

Ability (can), or a polite request (could).

Must / Have To

Structure: must / have to + base verb

You must wear a seatbelt.

“Must” often feels like the speaker’s own rule; “have to” feels more external.

Should / Ought To

Structure: should / ought to + base verb

You should see a doctor. / You ought to call her back.

Advice or a recommendation, not an order. “Ought to” means the same thing, just more formal and less common in speech.

Mustn’t vs Don’t Have To

Structure: forbidden vs optional

You mustn’t smoke here. (forbidden)  vs  You don’t have to come. (optional)

These look similar but mean opposite things: one bans it, the other just makes it optional.

May / Might

Structure: may / might + base verb

It may rain later. / You may leave the room.

Possibility (may/might are close to interchangeable here, might slightly less certain), or formal permission (may only).

Will

Structure: will + base verb

She will call you tomorrow. / I’ll get the door.

A prediction, a promise, or a decision made right at the moment of speaking.

Would

Structure: would + base verb

Would you like some tea? / I would help if I could. / We would go there every summer.

A polite offer or request, a hypothetical, or a repeated action in the past.

Shall

Structure: shall + base verb

Shall we dance? / Shall I open a window?

A formal suggestion or offer, mostly with I/we. Rare outside British English and formal writing.

Must vs Can’t (Deduction)

Structure: near-certain positive vs near-certain negative

She must be exhausted.  vs  That can’t be right.

Not obligation here, but logical certainty from evidence. “Must” concludes yes, “can’t” concludes no.

Should Have / Must Have

Structure: modal + have + past participle

You should have called me. / He must have left already.

A regret about what didn’t happen (should have), or a confident guess about what did (must have).