Modal Verbs of Obligation

Modal Verbs of Obligation in English: Must, Have to, Should (Clear Guide + Examples)

Every day, we tell people what they must do, what they have to do, and what they should do.

• You must stop at a red light.
• You have to pay your bills.
• You should get more sleep.

These small words carry big meanings: rules, duties, advice, expectations, and responsibility.

In English, we use modal verbs of obligation to talk about:

• what is necessary
• what is required
• what is recommended
• what is forbidden

They are everywhere in real life — in signs, rules, emails, school regulations, and work instructions — and they appear very often in Cambridge exams (B2 First, C1 Advanced) in Use of English, writing, and speaking tasks.

In this lesson, you will learn how to use the three most important modal verbs of obligation:

must – have to – should

You’ll also learn:

• when they mean the same thing
• when they are different
• and how to use them naturally and correctly.

Must

Meaning: Must expresses a strong obligation or something that is absolutely necessary.

We often use it for rules, strong instructions and things the speaker feels are very important.

• You must stop at the red light.
• Children must be quiet to avoid disturbing the neighbours.
• Everyone must obey the school rules.
• We must find a solution to this problem.

Negative: mustn’t

Mustn’t means it is forbidden or strongly not allowed.

Important: Do not use an auxiliary verb with must.

• You must tell the truth. You mustn’t
• Hurry up! We mustn’t be late.
• Watch your mouth! You mustn’t talk like that.

Tense limitation of must

Must is normally used only for the present and future.

To talk about the past or other tenses, we use have to instead.

• Sofia had to wear a uniform when she was in primary school.
• You’ll have to pay the bill — we don’t have any money.

Have to

Meaning: Have to is very similar to must, but it often expresses a general or external obligation (rules, laws, situations, circumstances).

A simple way to think about it:

must → what I think is necessary

have to → what the situation or rules make necessary

• You must speak English to be considered for this job. (speaker’s opinion / requirement)
• Elsa is a single mother, so she has to cook every day. (life situation)

No obligation: don’t have to / doesn’t have to

Don’t have to means it is not necessary (but it is not forbidden).

• You don’t have to knock — just come in.
• Liam doesn’t have to work at weekends.

Be careful:

mustn’t = forbidden

don’t have to = not necessary

Should

Meaning: We use should to give advice, recommendations, gentle obligations and opinions about what is the right thing to do. It is much weaker than must or have to.

• You should do your homework and study regularly.
• You should never judge people by their looks.
• Every citizen should obey the rules.

Negative: shouldn’t

• You shouldn’t buy that car unless you’ve tried it.
• You shouldn’t waste your time.

Quick comparison

must → strong obligation / rule / speaker’s authority

have to → obligation because of rules, laws, or situation

should → advice, recommendation, moral duty

Exam tip (B2–C1)

In Cambridge exams, these verbs are very common in Use of English transformations, Essays, reports, and reviews, as well as speaking tasks about rules, problems, advice, and responsibilities.

Knowing the difference between must / have to / should will instantly make your English more precise and more natural.

Download the exercise worksheet with modal verbs of obligation in PDF here

Learn about the modal verbs of deduction here

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