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Ch 1

Modals — Obligation and Necessity

Must, have to, need to, and their negative forms.

must, have to, mustn't, should, ought to
B1 Pre-intermediate

Modals: Obligation and Necessity

must, have to, should, ought to
Obligation / necessity
must, have to / has to
Prohibition
mustn't
No obligation
don't / doesn't have to
Advice
should, shouldn't, ought to

Must and Have to

You must be on time tomorrow.
Strong personal obligation or internal rule.
You have to wear a helmet on a bike.
External rule or law.
You mustn't park here.
Prohibition - it is forbidden.
You don't have to drive - we can walk.
No obligation - not necessary but allowed.

Should and Ought to

Advice - weaker than must / have to
You should take your jacket.
I think this is the right thing to do.
You ought to drink less coffee.
Same meaning as should - slightly more formal.
Should and ought to express advice, not laws or rules. Don't confuse with must.

Common mistakes

✗ I must go to work by bus yesterday.
Must has no past form. Use had to.
✓ I had to go to work by bus yesterday.
✗ You should to go home.
No to after modal verbs.
✓ You should go home.

Summary

must / have to
Obligation - something necessary
mustn't
Prohibition - something forbidden
don't have to
No obligation - not necessary but allowed
should / ought to
Advice - the right or wrong thing to do
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