Modal Verbs: Can, Could, Must, Should, May

What modal verbs do

Modal verbs express ability, possibility, permission, or obligation. They never change form (no -s, no -ing, no "to") and are always followed by the base verb.

Can / Could — ability and possibility

• Can — present ability or permission: I can swim. Can I leave early?
• Could — past ability, or a more polite request: I could swim when I was five. Could you help me?

Must — strong obligation or logical certainty

• Obligation: You must wear a seatbelt. (a rule)
• Certainty: She's not answering — she must be busy.

Note: must not means "forbidden," not "not necessary" — that's a common trap.

• You must not smoke here. (forbidden)
• You don't have to come. (not required, but allowed)

Should — advice and expectation

• Advice: You should see a doctor.
• Expectation: The bus should arrive soon.

Should is softer than must — it's a recommendation, not a rule.

May / Might — polite permission and possibility

• Permission (formal): May I ask a question?
• Possibility: It may rain later. It might rain later. (might is slightly less certain)

A quick comparison

Modal	Main meaning	Example
can	ability/permission	I can drive.
must	obligation/certainty	You must finish this.
should	advice	You should rest.
may/might	possibility/permission	It may rain.

Practice tip

Modal verbs are best learned in pairs of opposite strength: must vs should (strong vs soft obligation), can vs may (informal vs formal permission). Practicing the contrast, not the words alone, is what makes the difference stick.