How to Use Must in English - English Modal Verbs

Hi, I’m Oli.

Welcome to Oxford Online English!

In this lesson, you can learn about the verb 'must'.

'Must' can have many different meanings in English.

In this video, you can learn about the different meanings of 'must', and how to use 'must'

in English.

Part one, using 'must' to talk about obligations.

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 can't 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 probably in an examination hall, which of course is 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.

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.

Part two, 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."

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

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

Again, this means that it’s forbidden to use phones.

'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.

Part three: 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?

In a sentence like this, in a lesson, 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.

Part four: using 'must' to talk about logical necessity.

Finally, 'must' can be used to make deductions.

What are deductions?

Well, 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 directly.

Anyway, however you know, you know they’re rich.

"They must be rich" is a deduction.

You don’t actually know they’re rich.

You know something else; you know that they have a huge house and this makes you sure

that they’re rich.

Okay, 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 actually taken the exam.

You don’t know for yourself that it’s difficult.

However, you do know something else: you know that 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.

Okay, that's a strange point, so let's look at our next topic, which is negatives--part

five.

It’s important to remember that when you use 'must' with different meanings, the negative

(or opposite) word is also different.

Okay, let's look at this.

So, if 'must' is used for obligation, the opposite would be 'mustn't', or maybe 'can't'.

The opposite of 'mustn't' for prohibition would be 'must' or maybe 'have to'.

The opposite of 'must' for strong advice is 'shouldn't'.

And for making deductions, the opposite of 'must' would be 'can't', 'couldn't', or maybe

'must not'.

For example:

"Passengers must place bags above the seats."

Here, 'must' expresses an obligation.

The opposite would be:

"Passengers must not place their bags above the seats."

Or, if you wanted to explain this rule in spoken English, 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 the late afternoon."

Or perhaps:

"He must not be awake.

He never wakes up early."

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.

Part six: past forms.

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:

So, 'must' for obligation, the past form is 'have to'.

'Mustn't' for prohibition, the past form is 'couldn't'.

'Must' for strong advice or recommendations, the past form is 'should have'.

And for making deductions, the past form is '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 to talk about the past.

Let’s look at some examples:

"We must be there at 9.00 tomorrow."

This is an obligation.

In the past, you'd say, "We had to be there at 9.00 yesterday."

"We mustn’t smoke in the restaurant."

That's prohibition.

The past: "We couldn’t smoke in the restaurant."

"You must try the stew—it’s delicious!"

This is advice.

The past form: "You should have tried the stew—it was delicious!"

"They must be on the subway—I can’t get through on the phone."

That's a deduction.

The past form: "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'.

Okay, finally, let's do a 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 'must' means in each case.

Do you want more practice with this topic?

Check out the full version of the lesson on our website: www.oxfordonlineenglish.com.

The full lesson includes notes and exercises to help you practise what you’ve learned

in this class.

That’s the end of the lesson.

Thanks very much for watching!

I hope it was useful for you.

I'll see you next time!

Bye bye.