Name: 英語情態動詞 - 如何在英語中使用 "Must"？ (English Modal Verbs - How to Use 'Must' in English)
Uploaded: 2021-01-14T06:47:27.000Z
Duration: 14 min 27 s
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情態助動詞 A2

Hi, I’m Oli. Welcome to Oxford Online English! In this lesson, you can learn about the verb

基礎數量詞

'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

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

"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

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

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

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

"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

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,

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

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

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

"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.

"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.

"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

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

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

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