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What Are Modal Verbs for Advice, Obligation, and Prohibition?

Modal verbs are special verbs that add extra meaning to the main verb in a sentence. They show things like possibility, ability, permission, and in this lesson, advice, obligation, and prohibition.

These verbs always come before the main verb, which is usually in its base form (the infinitive without to).

• You should study. (Not You should to study.)
• He must go. (Not He must goes.)

How Do We Give Advice with 'Should' and 'Shouldn't'?

We use should and shouldn't to give or ask for advice. They express what is a good idea or a bad idea.

• Use should + base verb for positive advice. For example, You should get more sleep.
• Use shouldn't + base verb for negative advice. For example, You shouldn't eat so much sugar.

You can also use ought to instead of should. It means the same thing but is a bit more formal. For example, You ought to call your parents.

When Do We Use 'Must' and 'Have to' for Obligation?

Both must and have to express obligation, meaning something is necessary or required.

• We often use must for rules or laws that we personally agree with or feel are important. It often comes from the speaker's own feeling. For example, I must remember to send that email.
• We often use have to for rules or obligations that come from an external authority (like a school, a job, or the law). For example, We have to wear a uniform at work.

In many everyday situations, must and have to can be used similarly, especially when the obligation is clear. However, have to is more common in spoken English.

How Do We Express Prohibition with 'Mustn't'?

We use mustn't (or must not) to say that something is prohibited or not allowed. It expresses a strong negative obligation.

• For example, You mustn't use your phone during the exam.
• Another way to express prohibition is with can't or are not allowed to. For example, You can't park here. or Students are not allowed to leave early.

Remember, mustn't is very strong. It means it is forbidden.

What About 'Don't Have to' for Lack of Necessity?

When something is not necessary, we use don't have to. This is very different from mustn't.

• Don't have to means you can do it if you want, but it's not required. For example, You don't have to come to the meeting if you're busy. (It's optional.)
• Needn't (or do not need to) has the same meaning as don't have to. For example, You needn't worry about it.

Summary of Modal Verbs for Advice, Obligation, and Prohibition

Modal Verb	Meaning	Example
Should	Advice, recommendation	You should try this cake.
Shouldn't	Negative advice	You shouldn't worry so much.
Must	Strong obligation (often internal/speaker's feeling)	I must remember her birthday.
Have to	Obligation (often external rule/law)	All employees have to attend the meeting.
Mustn't	Prohibition, not allowed	You mustn't park here.
Don't have to	Lack of necessity, optional	We don't have to leave yet.

Examples

• You should visit the museum; it's fantastic!
• She shouldn't work so many hours without a break.
• What should I do if I lose my passport?
• All passengers must show their boarding pass.
• I have to finish this report by Friday.
• You mustn't touch the exhibits in the gallery.
• We don't have to pay for parking on Sundays.
• He needn't bring a gift to the party.

Practice

Common mistakes for Spanish speakers

Spanish speakers sometimes confuse mustn't with don't have to because in Spanish, a single verb form might cover both ideas of 'not allowed' and 'not necessary'.

• Incorrect: You mustn't work tomorrow. (If you mean 'it's not necessary to work')
• Correct: You don't have to work tomorrow. (It's not necessary.)
• Correct: You mustn't work here without permission. (It's forbidden.)

Another common error is using must for advice instead of should.

• Incorrect: You must try that new coffee shop. (If it's just a suggestion)
• Correct: You should try that new coffee shop. (It's good advice.)

Frequently asked questions

What is the main difference between 'must' and 'have to'?

While both express obligation, must often comes from the speaker's personal feeling or a strong internal conviction, like I must call my mum. Have to usually refers to external rules or obligations imposed by others, such as I have to work on Saturday.

Can I use 'ought to' instead of 'should'?

Yes, you can. Ought to has the same meaning as should for giving advice. It is slightly more formal and less common in everyday spoken English, but it is perfectly correct. For example, You ought to apologize.

How do I express 'not necessary' in the past?

To express lack of necessity in the past, you use didn't have to. For example, We didn't have to wear uniforms at our old school. You cannot use mustn't in the past tense for prohibition; instead, you would use weren't allowed to, as in We weren't allowed to use phones in class.

Why is understanding these modals important for the B1 Preliminary exam?

For the B1 Preliminary exam, you need to show you can express advice, obligation, and prohibition clearly. For example, in the Writing Part 2 (email or article), you might need to give advice to a friend or explain rules. In the Speaking Part 2 (long turn), you might describe a situation and suggest what someone should or must do. Correct use of these modals helps you achieve a higher score.

Related lessons

• Modal verbs
• Modal verbs
• Modal verbs
• Modal verbs