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Unit 10
Should, Must & Have To
Advice, obligation & necessity
Three Modals of Obligation
Should, must, and have to all express some degree of obligation or necessity — but they differ in strength and source. Like all modals, they are followed by the base form.
Should — Advice & Recommendations
Should is used for advice, recommendations, and expressing what you think is right or expected. It is a soft suggestion, not a strong command.
| Use | Example |
|---|---|
| Advice | You should see a doctor about that cough. |
| Recommendation | You should try the fish — it's excellent! |
| What is right / wrong | He shouldn't speak to her like that. |
| Expectation | The parcel should arrive by Friday. |
Must — Strong Obligation (Internal)
Must expresses strong obligation or necessity that comes from the speaker's own feeling or judgement. It is stronger than should.
| Use | Example |
|---|---|
| Strong obligation | You must finish the report today. |
| Logical deduction | She must be tired — she worked all night. |
| Prohibition (negative) | You mustn't smoke in here. |
Have To — External Obligation
Have to also expresses strong obligation, but the necessity comes from an external rule, law, or situation — not the speaker's own feeling. It uses do/does/did in negatives and questions.
| Type | Example |
|---|---|
| External obligation | I have to wear a uniform at work. (company rule) |
| No obligation | You don't have to come — it's optional. |
| Question | Do you have to work on Saturdays? |
| Past | She had to stay late yesterday. |
| He/She/It (present) | He has to leave by 6. |
Comparison: Should / Must / Have To
| Modal | Strength | Source | Example |
|---|---|---|---|
| should | Soft — advice/suggestion | Speaker's opinion | You should exercise more. |
| must | Strong — necessity | Speaker's own feeling | You must submit this today. |
| have to | Strong — necessity | External rule/law | I have to renew my passport. |
| don't have to | No obligation (optional) | External — it's a choice | You don't have to bring anything. |
| mustn't | Strong — prohibition | Rule or law | You mustn't park here. |
Must vs Have To in Real Life
- In positive sentences, must and have to are often both possible: I must / have to finish this today. The difference (internal vs external) is small.
- In the negative, they are completely different: mustn't = not allowed · don't have to = not necessary.
- Must is common in written rules and signs: Passengers must show their tickets.
Mustn't vs Don't Have To
These two look similar but mean very different things:
| Mustn't | Don't have to |
|---|---|
| = it is forbidden — do NOT do it | = it is not necessary — you can choose |
| You mustn't smoke here. (it's not allowed) | You don't have to wear a tie. (it's optional) |
Past and Future: had to / will have to
Must has no past or future form. Use have to:
| Time | Example |
|---|---|
| Past | I had to work late yesterday. (not |
| Past negative | We didn't have to pay — it was free. |
| Future | You'll have to wait a few minutes. |
| Question (past) | Did you have to wait long? |
Annotated Examples
Subject
Verb
Auxiliary
Object
Adjective
Adverb
Preposition
Hover over any word to see its role.
subjectYouauxiliaryshouldverbcallobjectherand apologise.
↳ Should = advice.
subjectIauxiliaryhave toverbwearobjecta uniformadverbat work.
↳ Have to = external rule.
subjectYouauxiliarymustn'tverbuseobjectyour phoneadverbwhile driving.
↳ Mustn't = strong prohibition.
Common Mistakes
You must to study.
→
You must study.
No "to" after modal verbs.
She musts leave early.
→
She must leave early.
"Must" never changes — no -s.
You don't must smoke here.
→
You mustn't smoke here.
Negative of must = mustn't, not don't must.
Do I must wear a tie?
→
Do I have to wear a tie?
"Must" doesn't use do/does in questions — use "have to".
You don't have to go — it's forbidden.
→
You mustn't go — it's forbidden.
"Don't have to" = optional. "Mustn't" = forbidden.
I musted leave early yesterday.
→
I had to leave early yesterday.
Must has no past form → had to.
You will must wait.
→
You will have to wait.
Two modals can't go together → will have to.