How to Use the Modal MUST - Learn English Grammar

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Hi, everybody. My name is Alisha.

Today, I'm going to talk about the words “must” and “must not” and how we use them to

talk about possibility.

In this lesson, I'm going to talk about a present possibility, so possibility now, and

I'm going to talk about a possibility in the past too.

So, with this lesson, I want to introduce a few common patterns and some example sentences

to show you the meanings of “must” and “must not,” for present and past.

And also, just to give you some tools so that you can use these grammar points to talk about

possibility in your life as well.

So, let's get started.

Okay, first, I want to talk about present possibility, the present possibility.

These are things we want to talk about now, we think are possible now.

The first group of examples, the first group of patterns I want to look at is “must be”

plus a noun phrase, “must be” plus an adjective or “must be” plus the verb in

the “-ing” form.

These are a few examples and some patterns we can use with “must” to talk about possibility

now, possibility in the present.

A couple points though, we use “must,” and “must” not as well, to make guesses.

We're making a guess about something we don't know in other words.

So, there's some information we do not have.

We're making a guess when we use “must.”

The nuance of “must” is that there's a very high chance something is correct.

So, very high meaning near 100%, a very high chance the guess is correct.

Also, keep in mind that because this is a guess, because you are kind of giving an idea

or giving a very strong opinion that something is probably correct, it's very strange to

use “must” and “must not,” for that matter, it's very strange to use them in most

cases to talk about yourself because we know ourselves and we know our group.

So, in a few exceptions, in a few special cases, we can use “must” and “must not”

to talk about ourselves but I'll talk about that later.

Just keep in mind that, in many cases, it is strange to use must and must not to talk

about yourself because you know yourself in most cases.

Let's take a look at a few examples of how to use these.

First, let's look at the first group, the first pattern here, “must be” plus a noun

phrase.

So, if we use “must be” plus a noun phrase, it talks about an expression that has a high

chance, something that has a high chance of being true and it's related to a noun phrase.

For example, the expression, “This must be the right house,” meaning there's a very,

very high near 100% chance that this house is the correct house.

In other words, this is the house I'm looking for, there's a very high chance of that.

One more example.

“She must be a student.”

Here, I've used “student” as my noun phrase.

So, in this situation, I'm looking at a person and I guess, based on her appearance, “Oh,

she must be a student.”

There's a high possibility she's a student.

These are a few examples of how to use “must be” with a noun phrase.

Let's look at another pattern.

The second pattern is “must be” plus an adjective.

So, just as we did with noun phrases, we use “must be” plus an adjective.

For example, “You must be tired,” looking at another person, you can guess, they must

be tired.

There's a very high chance that person is tired based on their appearance or some other

hints you get from them.

“Tired” is our adjective here.

Another one.

“They must be angry.”

“They must be angry.”

So, again, looking at a group of people or a couple of people, based on the appearance,

based on the situation, you guess there's probably a high chance those people are angry.

“They must be angry.”

Alright.

One more pattern.

I talked about using “must be” plus a verb in the “-ing” form.

Here, we see, again, “must be” but in this case, we have the “-ing” form of

a verb here.

So, in this example sentence, “The computer must be thinking.”

“Must be thinking,” “thinking” is a word we use to talk about like functioning

or moving or working, doing its job, in other words, but for computers, we like to sometimes

say, “The computer is thinking,” the computer is processing.

Here, a computer maybe is taking a long time, “The computer must be thinking,” there's

a high chance the computer is working, is thinking.

One more.

“He must be working,” maybe if you send your friend a message and there's not a response

in a timely fashion, the response is very slow.

You could say, to make a guess, “Oh, he must be working.

That's why he hasn't responded.”

“She must be working.”

There's a high chance that at this point in time, at this point now, that person is doing

something, in this case, working.

Okay.

Let's take a look now at the negative examples of this grammar point.

So, “must not” plus the same points we just talked about.

“Must not” plus a noun phrase, “must not” plus an adjective and “must not”

plus a verb in the “-ing” form.

So, we'll use exactly the same pattern and we can even use, in many cases, similar noun

phrases and similar adjectives.

But, it's important to remember “must not” expresses extremely low possibilities.

So, extremely low possibility meaning very near to zero if not impossible.

So, the nuance here is it's impossible or there's like a 0.001% chance.

Very, very low possibility.

So, please keep this in mind.

“Must be” or “must” is used to make a high possibility guess.

“Must not,” however, expresses very low possibility, very low.

So, let's look at a few examples similar to the other examples we looked at.

With a noun phrase.

“This must not be,” again, “must not be the right house.”

So, it is impossible that this house is the correct house, in other words.

Here, “She must not be a student.”

Here, high chance she is a student.

Here, low chance she is a student, impossible she is a student.

Next one, with adjectives here.

“He must not be busy,” in this case.

“He must not be busy” is my adjective.

Low chance he is busy, very low chance he is busy.

“They must not be happy,” “They must not be happy,” is a very low chance that

they are happy.

So, again, these are all guesses based on some outside information like a visual or

some other hints you have.

You’re making a guess, in this case, a very low chance of something.

To move on to the final pattern, “She must not be studying.”

So, at this point in time, there's a very low chance she is studying, very low chance.

“She must not be studying.”

One more.

“They must not be traveling,” here.

So, again, low chance that these people in the sentence are traveling.

So, please just keep in mind, positive statements express high possibility, negative statements

express low possibility or near impossibility.

Okay, now, let's take a look--let's move from present possibility to past possibility.

This gets a little bit more complicated or a little bit more challenging, maybe.

But, we can keep many of the same patterns really.

When we want to talk about past possibility, we need to use a pattern like “must have

been.”

So, you can see in present possibility, we use “must be,” in past possibility, for

a positive sentence anyway, we use “must have been.”

This is a grammar change so please be careful.

But, we can use noun phrases, adjectives and the same verb in the “-ing” form just

as we did with present possibility.

This is true for positive statements, here, and for negative statements.

Again, we update negative statements with “must not have been” in the past.

So, let's take a look.

Again, we're looking at very high chances with the positive.

However, they’re in the past.

These are very high chance situations, very high chance of guesses in the past, in this

case.

And, as we talked about in the present possibility section with “not,” “must not have been,”

this expresses impossibility in the past.

Impossibility, very, very low chance.

So, let's look at a few examples here too.

For positive statements, “That must have been the right house,” in the past.

“Ah, that must have been the right house,” for example, you drive past the correct place,

you can remark, “Oh, that must have been the right house.”

Another one, “He must have been the waiter.”

“He must have been the waiter.

Maybe I made a mistake.

Oh, he must have been the waiter,” for example.

With an adjective, “The computer must have been broken.”

“The computer must have been broken.

That's why it wasn't working,” for example.

Or, “You must have been tired,” “You must have been tired after your trip,” for

example.

So, again, making a very high-level possibility guess in the past.

Here, with the verb form in the “-ing,” “She must have been studying,” “She

must have been studying,” for example, “last night,” or, “this morning.”

“He must have been working,” as well.

So, you can see, I've just updated some of these to show how we changed the sentence

from the past to the present and vice-versa.

Let's look at some negative examples.

“That must not have been the office,” so, in other words, there was a mistake, a

low chance that that building was the office.

Another example.

“You must not have heard the news,” there's a very low chance you heard the news.

So, here, “must not” expresses that very low chance.

Okay.

Just a couple more patterns I want to talk about.

Over here, I have “must have” plus a verb and “must not have” plus a verb.

So, these are a couple of different patterns we can use in the past.

But, we use a verb with them.

Instead of using a noun phrase and adjectives and so on, as we've talked about here, we

use just a regular verb in the past tense here.

But, again, in the positive form, positive form expresses a high chance of something

high-level possibility of something, low chance for the negative form, as well, so impossibility.

So, let's take a look then at how we use a verb after “must” and “must have”

in the past.

So, “We must have made a mistake,” this is one of those cases where “we,” using

“we” or using yourself as the subject of a sentence is actually okay because you're

talking about yourself in the past and something that is different between now and the past.

In this case, you learn some new information and you make a guess about your past self.

In this case, I made a mistake but, in the past, given new information, you can guess,

“Oh, we must have made a mistake.”

“I must have made a mistake.

I didn't know it at the time.”

In this case, we use, “He must have made a mistake.”

So, here's the past participle form of the verb, “must have made a mistake” or “they

must have gone to somewhere else,” or “they must have whatever.”

We can use the past participle form of the verb here to show a past activity, a past

possibility situation.

Let's look at one more example then.

“My package must have shipped.”

So, in this case, the package, there's a high possibility my package was sent from a warehouse,

for example, or from a company.

There's a high level of possibility here.

“My package must have shipped.”

That past participle form of the verb.

Okay, let's look, however, at the negative form.

“He must not have come to work,” is my first example.

“He must not have come to work,” a very low chance, impossible he came to work.

So, there's a very, very low chance this person came to work.

“He must not have come.”

Again, this past participle form of the verb here.

Finally, “She must not have forgotten.”

You could say, “She must not have forgotten the report,” “She must not have forgotten

her keys,” or just, “She must not have forgotten some other information.”

So, again, past participle form of the verb is used here.

“She must not have forgotten blah, blah, blah.”

We can add some extra information about what she must not have forgotten.

So, low possibility she forgot, extremely low.

So, please keep in mind that when we use the positive form, it expresses near 100% possibility

and with a negative form near 0% possibility, near impossible.

So, in this lesson, I've introduced present possibility example sentences and past possibility

example sentences.

You'll notice here I haven't introduced any future tense possibility.

That's because, in most cases, we do not use “must” to talk about future possibility.

In those cases, we use words like “probably,” like, “I'm probably going to,” or, “I'll

probably blah, blah, blah.”

So, we don't generally use “must” to talk about the future possibility.

There are a few cases where we may use the word “must” but maybe I can talk about

that in a different video.

For now, I'd like to just focus on present and past possibility with a few examples here.

So, I hope this lesson was useful for you.

If you have any questions or if you want to try to make a sentence, please feel free to

do so in the comments.

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