must, have to, should

Three modals for obligation and advice — each with a different level of strength.

must

Strong internal obligation — I feel this is necessary: I must call her.

have to

External rule or requirement — it comes from outside: I have to wear a uniform.

should

Advice or recommendation — a good idea: You should see a doctor.

must vs have to

Internal obligation vs external rule

must — I decide

I must study tonight.

my own decision

You must try this restaurant!

strong recommendation

have to — rule

I have to wear a helmet.

law/rule from outside

She has to start at 9.

company rule

In everyday spoken English, have to and must are often interchangeable. The distinction matters most in formal writing.

Negatives: the critical difference

mustn't vs don't have to — opposite meanings!

mustn't = prohibited

You mustn't park here.

= it is NOT allowed

You mustn't tell anyone.

= forbidden

don't have to = not necessary

You don't have to come.

= optional, up to you

You don't have to hurry.

= there's no rush

Critical: mustn't = FORBIDDEN. Don't have to = NOT REQUIRED (but OK if you want). Confusing these changes the meaning completely.

should / shouldn't — advice

Weaker than must — a good idea, not an order

Advice

You should eat more vegetables.

Recommendation

You should see that film — it's brilliant.

Negative advice

You shouldn't stay up so late.

Asking for advice

Should I call her?

ought to

Ought to = same meaning as should (more formal).

had better — urgent advice

Stronger than should — implies a bad consequence

had better + base

You had better hurry — the train leaves in 5 minutes!

Contraction

You'd better call her now.

Negative

You had better not be late.

"Had better" always takes the base verb (not "had better to go"). And "had" here is not past — it refers to the present/future.

Common mistakes

You mustn't wear a uniform. (meaning: it's not required)

→

You don't have to wear a uniform.

mustn't = forbidden · don't have to = not required

You should to see a doctor.

→

You should see a doctor.

Modals never take "to"

She musts go now.

→

She must go now.

Modals never take -s

You don't must smoke here.

→

You mustn't smoke here.

negative of must = mustn't (never don't must)

Your turn

Answer out loud — full sentences

Give rules and advice with must, have to and should.

What do you have to do every day?

What mustn't people do on a plane?

Your friend is always tired. What should he do?

What don't you have to do at the weekend?

Recap

must

strong internal obligation

I must study.

have to

external rule/requirement

I have to wear a uniform.

should

advice / recommendation

You should see a doctor.

mustn't

FORBIDDEN

You mustn't park here.

don't have to

NOT REQUIRED

You don't have to come.

Practice now →

Must & Should