1. Using Must to Talk about Obligations and Rules

• I must remember to call her—it’s her birthday.
• All passengers must hold a valid ticket.
• You must be here on time, otherwise we’ll miss the start.
• I must finish this work today.

2. Using Must to Talk about Prohibition

• You mustn’t smoke here.
• Mobile phones must not be used while the plane is flying.

3. Using Must to Give Strong Advice

• You must read this book—it’s amazing!
• If you go to Paris, you must take a walk by the river at night.
• We must hire some new staff as soon as possible.

4. Using Must to Make Deductions

• They must be rich. Their house is huge!
• It must be a difficult exam—only 10% of candidates pass.
• It must have rained last night.

5. Negatives with Different Meanings of Must

Meaning	Negative form of must
Obligation	mustn’t, can’t
Prohibition (mustn’t)	must, have to
Strong advice/recommendation	shouldn’t
Making deductions	can’t, couldn’t, must not

6. Past Forms of Must

Meaning	Past form of must
Obligation	had to
Prohibition (mustn’t)	couldn’t
Strong advice/recommendation	should have
Making deductions	must have

1. Using Must to Talk about Obligations and Rules

First, you can use must to talk about obligations and rules. For example:

• I must remember to call her—it’s her birthday. –> This is an obligation. It’s very important to me.

• All passengers must hold a valid ticket. –> This is a rule. You cannot choose to travel without a ticket.

These are things that it is necessary or important to do. If you must do something, you have no choice.

When you use must for obligations, it can mean that the obligation is important to you personally.

It’s not just a rule that someone else made; using must means that something matters to you. For example:

• You must be here on time, otherwise we’ll miss the start.

Using must shows that you care about whatever you’re going to see.

Maybe you’re going to see a concert that you’ve been looking forward to for ages, and you’ll be really unhappy if you miss even a minute of it.

• I must finish this work today.

Again, this means that I feel strongly about this. Maybe I promised someone that I would finish it by today, and it’s important to me that I keep my promise.

When talking about rules, we mostly use must in formal or written English. For example:

• All candidates must show valid ID. –> This is a rule in an examination hall, a formal setting.

• Employees must wash their hands before working with food. –> This could be from a sign in a restaurant or café. Again, this is more likely to be written than spoken.

In informal English, we mostly use the verb have to to talk about rules and obligations. You can learn more about the difference between must and have to in this lesson from Oxford Online English.

To review, you can use must to express an obligation if the obligation is important to you personally, or if you want to sound more formal.

2. Using Must to Talk about Prohibition

You can use mustn’t to say that something is not allowed or forbidden. This is the opposite of using must to talk about obligations or rules, and is similar in meaning. Look at some examples:

• You mustn’t smoke here.

This means that smoking is forbidden; it’s against the rules to smoke here.

• Mobile phones must not be used while the plane is flying.

This means that it’s forbidden to use phones.

Again, must expresses the idea that you don’t have a choice. If you mustn’t smoke here, then it’s not OK to smoke, and you don’t have a choice.

Mustn’t with this meaning is similar to can’t.

With obligations and rules, we mostly use must in formal or written English, and have to in informal English, as we said before.

However, you can use mustn’t in both formal and informal English.

3. Using Must to Give Strong Advice

Must can be used to give someone advice or a recommendation.

For example:

• You must read this book—it’s amazing!
• If you go to Paris, you must take a walk by the river at night.
• We must hire some new staff as soon as possible.

In these examples, must has a strong meaning. If I say:

• You must read this book—it’s amazing!

I don’t mean that you have an obligation to read it. I mean that I really, really recommend this book. In the same way, if I say:

• If you go to Paris, you must take a walk by the river at night.

This is advice or a recommendation. It doesn’t mean that it’s a rule or an obligation.

Sometimes, the meaning of must depends on the context. For example:

• We must hire some new staff as soon as possible.

Must here could mean that hiring new staff is necessary, and it’s an obligation, or it could be a strong recommendation.

How do you know what must means?

In a sentence like this, where there’s no context, you can’t say exactly what must means. In real life, the meaning would depend on the context: who’s speaking, what the situation is, and so on.

When you use must in this way, you can’t use the negative. There’s no way to use mustn’t to give advice or make recommendations. Only the positive form—must—can have this meaning.

4. Using Must to Make Deductions

Finally, must can be used to make deductions.

What are deductions?

Look at some examples, and then I’ll explain what this means:

• They must be rich. Their house is huge!
• It must be a difficult exam—only 10% of candidates pass.
• It must have rained last night.

Take the first sentence:

• They must be rich. Their house is huge!

Do we know they’re rich? No, we don’t know for a fact.

But, we do know that they have a huge house. Huge houses cost a lot of money. Therefore, they must be rich. We’re sure that they’re rich, because otherwise they couldn’t afford such a big house.

Compare:

• They must be rich.
• They’re rich.

What’s the difference?

They’re rich is a fact. We know they’re rich. Maybe you’re their accountant, and you know all about their financial situation. Maybe they told you how much money they have. Anyway, you know they’re rich.

They must be rich is a deduction. You don’t actually know they’re rich. You know something else (they have a huge house) and this makes you sure that they’re rich.

Let’s look at another example:

• It must be a difficult exam—only 10% of candidates pass.

Again, using must expresses a deduction.

If you say this, you haven’t taken the exam. You don’t know for yourself that it’s difficult. However, you do know something else (only 10% of candidates pass), and this makes you sure that it’s a difficult exam.

In our last example:

• It must have rained last night.

Did you see it rain? No, you didn’t. But, you’re sure it rained. How?

Probably, you can see that the ground is wet. You didn’t see or hear the rain, but you can make a deduction from what you see. What you see makes you sure that it rained last night.

You can also use must not with this meaning, but you can’t use the short form mustn’t in standard English to express the idea of deduction. Mustn’t is only used to say that something is forbidden.

5. Negatives with Different Meanings of Must

It’s important to remember that when you use must with different meanings, the negative (or opposite) word is also different.

Meaning	Negative form of must
Obligation	mustn’t, can’t
Prohibition (mustn’t)	must, have to
Strong advice/recommendation	shouldn’t
Making deductions	can’t, couldn’t, must not

For example:

• Passengers must place bags above the seats.

Here, must expresses an obligation. The opposite is:

• Passengers must not place their bags above the seats.

If you wanted to explain this rule in speech, you might say:

• You can’t put your bag above your seat.

Another example:

• You mustn’t say anything to her.

Here, must expresses prohibition. The opposite is:

• You must say something to her.

Or:

• You have to say something to her.

Finally, if you say:

• He must be awake. It’s two o’clock in the afternoon!

Here, must expresses a deduction. The opposite could be:

• He can’t be awake. He never gets up before midday.

Or perhaps:

• He must not be awake. He never gets up before midday.

Remember that when making deductions, you can’t use the contraction mustn’t. You need to use the full negative form: must not.

It’s important to remember that these opposites are not all the same! For example, if you use must to express prohibition, the possible opposites are must and have to. This doesn’t mean must and have to have the same meaning.

Modal verbs are complicated. For now, you need to remember one important point:

Different meanings of must have different negatives.

OK? Good! Let’s look at one more point on how to use must in English.

6. Past Forms of Must

Like with negatives, if you want to use must to talk about the past, the past form depends on the meaning of must.

Often, you need to use a different verb. Take a look:

Meaning	Past form of must
Obligation	had to
Prohibition (mustn’t)	couldn’t
Strong advice/recommendation	should have
Making deductions	must have

Using must with a past meaning is only possible if you are using must to make deductions. In all other cases, you need to use a different verb.

Let’s look at some examples:

• We must be there at 9.00 tomorrow. (Obligation) –> We had to be there at 9.00 yesterday.

• We mustn’t smoke in the restaurant. (Prohibition) –> We couldn’t smoke in the restaurant.

• You must try the stew—it’s delicious! (Advice) –> You should have tried the stew—it was delicious!

• They must be on the subway—I can’t get through on the phone. (Deduction) –> They must have been on the subway—I couldn’t get through on the phone.

You can see how you need to use different verbs to talk about the past, depending on the meaning of must.

7. Review

Let’s review what we’ve studied in this lesson:

Must can be used to express obligation and prohibition, to give strong advice, and to make deductions.

In some cases, must is similar to other verbs. For example, must used to express prohibition is similar to can’t. However, remember that similar does not mean the same! Must is unique, and no verb is ever exactly the same as must.

Different meanings of must have different negative and past forms. In other words, the way you use must depends on the meaning of must. You can’t just use must in the same way in every sentence; you need to think about what it means in each case.