Published at February 17, 2025

Course on Using Modals to Express Obligation - TOEIC® Preparation

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Whether to express a personal obligation, imposed by rules or circumstances, modals play an essential role in daily, professional, or academic communication. This course explores the main modals must, have to, and shall, as well as alternative expressions like need to and be supposed to, to help you choose the most appropriate expression according to the context.

1. "Must" for Expressing Obligation

A. "Must", When to Use It?

Must is the most common modal to express an obligation. It is used to express:

A strong personal obligation: the speaker thinks it is absolutely necessary.
- You must stop smoking for your health.
An obligation that comes from oneself, not from rules or others.
- I must finish this report before tomorrow.

B. "Must not" for Expressing Prohibition

The negative form of "must" is "must not" (or "mustn't"), which expresses prohibition or the obligation not to do something.

You mustn't open this door under any circumstances.
He mustn't disclose that information.

For more information, you can read our course on modals - expressing prohibition

C. "Must" in Interrogative Form in Formal Language

Although rare, it is possible to use "must" in the interrogative form, mainly in formal or elevated language:

Must I really attend the entire conference?

2. "Have to" for Expressing Obligation

A. "Have to" and "Has to", When to Use Them?

"Have to" (and "has to" in the third person singular) is used to express:

An obligation that comes from outside, like a rule or a situation that forces us to do something.
- I have to renew my passport soon.
Something necessary in a general or factual way.
- She has to call her boss before 6 p.m.

B. "Must" or "Have to", Which to Use?

Based on Meaning

"Must" and "have to" both translate to "devoir" in French, but they have different meanings. Here is the main difference:

"Must" expresses an obligation that comes from the speaker (it's their choice or opinion)
"Have to" expresses an obligation that comes from outside (like a law, rule, or situation)

Based on the Tense of the Sentence

Unlike "must", which can only be used in the present, "have to" can be conjugated in all tenses.

Tense	Form of "have to"	Example with "I"
Past	had to	I had to wake up early yesterday.
Present	have to / has to	I have to leave now; I'm running late.
Future	will have to	I will have to prepare for the exam tomorrow.
Present perfect	have had to has had to	I have had to change my plans because of the rain.
Conditional	would have to	I would have to leave early if the train is late.
Past conditional	would have had to	I would have had to take a taxi if the bus hadn't arrived.

C. "Don't have to" for Expressing Absence of Obligation

We use "don't have to" not to express prohibition, but rather to express absence of obligation.

You do not have to come if you're busy.
He doesn't have to work on weekends.

For more information, you can read our course on modals - expressing absence of obligation

3. Shall for Expressing Strict Obligation

In official documents, contracts, laws, or regulations, shall is commonly used to express a strict obligation. In this context, it has a force similar to must.

The tenant shall pay the rent on the first day of each month.
Employees shall comply with the company's code of conduct.

In a formal or old-fashioned register, shall can also express an obligation, but it is often replaced by must or have to in modern English.

You shall not pass.
Each participant shall bring their own equipment.

The use of the pure modal "need" is quite rare, mainly used in a formal register in the negative or interrogative form. Generally, the semi-modal "need to" is used instead.

Need I really apologize?
You needn't worry.

"Need to" is used to show that there is a necessity or a need to do something. "Need to" is very close to "have to", but it is often more personal.

It expresses something one must do because of a situation or because it is logical, rather than because a rule or law imposes it.

I need to buy groceries; the fridge is empty.
She needs to prepare for her exam tomorrow.

B. "Don't need to" for Expressing Absence of Obligation

We can use "do not need to" (or "don't need to") to express absence of obligation or necessity.

You don't need to hurry; we have plenty of time.
He doesn't need to attend the meeting if he's busy.

For more information, you can read our course on modals - expressing absence of obligation

6. "Should" / "Ought to" for Expressing Obligation

A. "Should" and "Ought to", When to Use Them?

"Should" and "ought to" are used to express:

An advice or a recommendation.
Sometimes, a mild obligation or a strong suggestion.

Even though these two semi-modals have a very close meaning, their difference lies in the fact that "ought to" is more formal than "should".

You should see a doctor if the symptoms persist.
They ought to be more careful with their spending.

B. "Should not" and "Ought not to" for Recommending Not to Do Something

We use these two semi-modals to express a recommendation to avoid doing something.

In essence, they are used to give the advice not to do something.

Just like their affirmative version, ought not to and shouldn't have a very close meaning, but ought not to is much less common. It is often considered a bit old-fashioned or too formal, and is not used much today.

You shouldn't eat so much sugar.
We shouldn't ignore the warning signs.
You ought not to be so rude to your parents.

C. "Should" / "Ought to" OR "Must" / "Have to", Which to Use?

Generally, we use should to advise, and must or have to to impose a strict obligation. Here is a brief summary:

Should / ought to: Used to express a mild obligation or advice. They suggest an action, but without mandatory character.
- You should drink more water to stay healthy.
Must / have to: Used to indicate a strong obligation or a strict necessity, imposed by a law, a rule, or an urgent situation.
- You must wear a seatbelt while driving.

7. "Be supposed to" for Expressing a Mild Obligation

The phrase "be supposed to" is often used to indicate what is expected in a situation, without being as strong as "must" or "have to".

We are supposed to hand in the report by Friday.
He is not supposed to park here.

8. "Be to" for Expressing a Formal and Official Obligation

The phrase "be to" is used in a very formal register or in official announcements, to express an obligation or an official plan.

The President is to visit the factory next Monday.
All students are to attend the ceremony.

9. Conclusion

On the day of the TOEIC®, it is important to master these modals on obligation, as they will be used either in texts and audios — you will need to understand the context — or directly in fill-in-the-blank sentences.

Summary of Modals Expressing Obligation

Modals/Structures	Source of Obligation	Strength of Obligation	Example
Must	Internal (speaker, moral authority)	Strong (subjective)	I must finish this task now.
Shall	Legal authority, formal directive or commitment	Strong (formal or legal)	Legal directive: All employees shall comply with the company's code of conduct. Formal commitment: You shall receive the package within 3 days.
Have to	External (law, rules, circumstances)	Strong (objective)	Law: I have to pay my taxes by April 15th. Rules: You have to wear a helmet when riding a bike. Circumstances: I have to take an umbrella; it's raining heavily.
Need to	Internal or external (need)	Strong (necessity)	Internal: I need to sleep early tonight; I'm exhausted. External: You need to submit the form before the deadline.
Should / Ought to	Internal (advice, recommendation)	Medium (advice, moral)	Advice: You should visit the doctor if you feel unwell. Recommendation: We ought to save more money for emergencies.
Be supposed to	Social norm or expectation	Moderate (expected)	Norm: Students are supposed to be quiet in the library. Social expectation: You are supposed to RSVP for the wedding invitation.
Be to	Formal, official context	Strong (in an official context)	The Prime Minister is to address the nation tomorrow evening. The students are to meet their teacher at the museum at 10 a.m.

Key Points to Remember on Modals Expressing Obligation

Here is a summary of the key points to remember about modals expressing obligation:

Must vs. Have to:
- Must: strong obligation, subjective, coming from the speaker (or an internal authority).
- Have to: strong obligation, but imposed by a rule or an external circumstance.
Shall is used in formal contexts, legal or for official directives. In everyday English, it is less frequent and often replaced by must.
The negative form of must (mustn't) means "prohibition", while the negative form of "have to" (don't have to / doesn't have to) means "not obliged to...".
Need to is close to "have to" but often highlights a personal or practical need.
Should / Ought to: advice or mild obligation, less strong than "must" or "have to".
Be supposed to: expresses what is expected or required according to a rule or convention, without being as strict as "must" or "have to".
Be to: a more formal usage, often in official and legal contexts, to express a plan or an obligation.

Other Courses on Modals

For more information on modals, you can read our various courses on the subject: