Overview
Use should / ought to for advice, have to / must for obligation or necessity, and don’t have to for a lack of necessity. Be careful: don’t have to ≠ must not.
1) Advice: should / ought to
We give recommendations and good ideas. Ought to is less common and a bit more formal; meaning is the same as should.
You should take your dog for a walk every day.
New owners ought to research breeds before adopting.
Form
subject + should/ought to + base verb
She should see a vet. • You ought to clean the litter box.
2) Obligation/Necessity: have to / must
Use for rules, laws, and strong need. Must sounds stronger and is common in signs and rules.
You must keep dogs on a leash in this park. (rule)
I have to buy cat food after work. (necessity)
Form
subject + have/has to + base verb • subject + must + base verb
He has to pay the adoption fee. • Visitors must wash hands before entering the kennel.
3) Lack of Necessity: don’t have to
Use when something is not necessary, but optional. It does not mean prohibition.
You don’t have to buy a purebred; a rescue is fine.
She doesn’t have to come early; the clinic opens at 9.
Compare with must not (prohibited)
You must not leave the gate open. (prohibited)
You don’t have to stay for the whole class. (optional)
Quick Summary
| Function | Modals | Meaning | Example |
|---|---|---|---|
| Advice | should / ought to | good idea / recommendation | You should book a vet visit. |
| Obligation / Necessity | must / have to | required by rule, law, or need | Pets must get vaccinations. |
| Lack of necessity | don’t have to | optional; not required | You don’t have to buy new toys every week. |