Modal Verbs Untangled

Can, could, must, might, should, shall, will — nine small words that carry huge shades of meaning. Here's how to pick the right one.

🎯 Ability, Permission & Possibility

✅ can / could

Can = present ability/permission. Could = past ability, OR a polite/possible version of "can".

She can speak French. Could you pass the salt? I could swim at age five.

🤔 may / might

May = formal permission or possibility. Might = weaker, more uncertain possibility.

You may leave now. It might rain later.

✅ must — strong obligation

A rule or an obligation the speaker feels personally (internal), or a confident deduction.

You must wear a seatbelt. She must be tired. (deduction)

❌ mustn't vs don't have to

Mustn't = forbidden. Don't have to = no obligation, but allowed. These are opposites, not synonyms!

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

👤 have to — external obligation

An obligation imposed by someone/something else (a rule, a boss, a law) — not the speaker's own feeling.

I have to finish this by 5pm. (my boss said so)

💡 should — advice, not obligation

Weaker than must/have to. A recommendation, not a rule.

You should see a dentist.

🤝 shall — offers & suggestions

Used with I/we to offer or suggest something. Rare with other subjects.

Shall we order dessert? Shall I open the window?

✅ will — certainty, promises

Future facts, promises, and spontaneous decisions.

I will call you tonight. It'll be fine, I promise.

Expectation vs reality

Be supposed to describes what's expected/planned — often used when reality doesn't match the plan.

The meeting was supposed to start at 9. (but it didn't) You're not supposed to eat here. (against the rules)

Modal	Main use	Example
can	Ability / informal permission	She can swim.
could	Past ability / polite request	Could you help me?
may	Formal permission / possibility	You may go.
might	Weak possibility	It might rain.
must	Strong obligation / deduction	You must be joking.
have to	External obligation	I have to work today.
should	Advice	You should rest.
shall	Offer / suggestion (I/we)	Shall we go?
will	Future fact / promise	I will help.