Modal Verbs | Must Should (Have to) | English Grammar Lesson | B2-Upper Intermediate

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Hello and welcome everyone to this lesson which  is based on a question that Elke asked me,  

and so let's see what the question is. Elke has  asked: 'Is 'must not' the opposite of 'must'?  

Surprisingly, the answer is no! The modal verb  'must' has two main functions, and in both cases,  

'must not' is not the opposite. So, let's see  what these two functions are, and how we express  

their opposites. So, let's start with the most  common function of the modal verb 'must', which  

is 'necessity'. So, to express a necessity to do  something or not to do something, we use 'must',  

and 'must' works both for the present tense  and for the future tense. So, let's look at  

the examples: I feel exhausted. I must have a  break now. It is necessary that I have a break  

now. The fridge is empty. We must go grocery  shopping tomorrow. This is used for the future.  

The verb is still 'must', and it's absolutely  necessary to do the shopping tomorrow. But also,  

if you want to express a necessity not to do  something, we stay with 'must'. So, you could say:  

I feel exhausted. I must not continue working.  Or: The fridge is empty. We must not forget to  

order groceries tomorrow. So, 'must' for a  necessity to do or not to do something. So,  

what is the opposite then? If 'must not' is not  the opposite, what is the opposite? How do we  

express that something is not necessary? So, to  remove a necessity, we use 'not have to' or 'not  

need to'. We have two options here, for example:  I feel fine. I don't have to have a break now.  

It's not necessary. Or: I don't need to have a  break now. And the other example: There's enough  

food in the fridge. We don't have to or we don't  need to go grocery shopping tomorrow. Right, so  

usually after we understand this, that 'must' is a  necessity, negative or positive, and to remove it,  

we have to use 'don't have to' or 'don't need  to', another question comes up which is: So,  

can we use 'have to' instead of 'must' also  in the positive form for necessity? Now,  

strictly speaking, grammatically speaking, 'must'  expresses a specific necessity felt by the speaker  

whereas 'have to' expresses an external obligation  or rule. But, in spoken English, this is the good  

news, in spoken English, 'must', 'have to', and  'need to' are often used interchangeably in the  

positive sense. So, the best word to have used for  the example 'the fridge is empty' is 'we must go  

grocery shopping tomorrow' because it's a specific  necessity for the speaker of this sentence. It's  

not an obligation or a rule, right? But, in  spoken English, you can hear any of these verbs:  

'we must go', 'we will have to go', 'we will  need to go'. Now, let's take another example  

where we have an external rule, where we should  really use 'have to' or 'need to'. For example:  

During take-off and landing, you have to /  you need to fasten your seat belt. But again,  

because it's a positive idea, we can use 'must'  as well. So, that's good news. You don't have  

to worry too much about 'Is it an external  obligation? Is it a rule? If it's a necessity,  

whether it comes from you or from outside, you  can use any of these three forms. So, that's the  

good news. The next question that usually comes up  is: 'Okay. So we said 'must' can only be used for  

the present and future tenses. How do we express a  necessity in the past? So remember that we had two  

types: a positive necessity to do something  or negative. For the positive, it's easy,  

right? 'must' has no past form. So, we use the  synonyms 'have to' and 'need to'. So, we say:  

I felt exhausted. I had to have a break. I needed  to have a break. Or: The fridge was empty. We had  

to go grocery shopping the following day. Or: We  needed to go grocery shopping the following day.  

So, that's straightforward. But, how do we now  put a negative, the 'must not' necessity, into  

the past? Well actually, there isn't a specific  verb or expression that we can use for a negative  

necessity in the past. We just have to describe  the situation. We have to change our sentence.  

For example, if in the present, we had: I'm  exhausted. I must not continue working. Expressing  

a personal necessity not to do something. For  the past, we have to use 'had to'. It's only,  

the only thing that expresses a necessity, and we  have to change our verb: I must not continue. So:  

I had to stop. Right? We make the actual the main  verb the opposite. If it's a rule, an obligation,  

excuse me, an external rule, for example,  a parent has said to someone: You must not  

speak to strangers. To put this into the past, we  just have to describe the situation. It's a lack  

of permission: 'you must not'. So, we can say: We  were not allowed to speak to strangers. Right. So,  

I think that's it for, yes, for necessity. Let's  look at the second function of 'must', which is  

'assumption'. What is important to remember is  that this assumption you have has to be based on  

evidence, for example .. And then we use 'must' to  assume that something is true based on evidence.  

For example: John's passport is not here, so he  must be abroad. The only thing we can deduce,  

conclude, assume, is that he is abroad with  his passport. Or: Jill's car is not outside.  

She must have gone shopping. So again, we think:  Okay, so if the positive assumption is 'must',  

a negative one, that something is not true, should  be 'must not'. But, it isn't. In order to express  

that something is not true, not possible  based on evidence, we have to use the modal  

verbs 'cannot' and 'could not'. So, if we now have  the opposite situation: John's passport is here,  

so it cannot be true that he's abroad. It's not  possible: He cannot or could not be abroad. Or,  

the other example, which was in the past: Jill's  car is outside, so it cannot be true that she has  

gone shopping: She cannot / could not have gone  shopping. Right so let's, just before we end this,  

that's the end of 'must', 'have to' situation and  assumption, but often I also get the question:  

'Is there a difference between 'must'  and 'should'. I hear, occasionally,  

my students using them interchangeably . It's best  to avoid that. They don't give the same idea. You  

need to use 'should' to give advice, and advice  is not expressing a necessity or obligation. So,  

let's compare the feeling of these two sentences.  if I say ... Our first example was: I feel  

exhausted. I must have a break now. 'must' was the  best choice, and we said you can also use 'have  

to' or 'need to'. This is strong. When you say 'I  must have a break.' or 'I have to have a break.'  

It means that if I don't, something negative may  happen. There might be a negative consequence,  

like I might get a really bad headache. I  must stop. Right? It's not negotiable. But,  

if you now just say: Oh I feel exhausted. I should  have a break. That does not have that idea at all.  

It doesn't mean it's necessary. It's just a good  idea maybe to have a break, but oh well, okay,  

I can also continue working. So, make sure that  you remember this difference and use it correctly.  

Otherwise, your listener will get a different idea  from you. Another example would be, for example  

for the external obligation or rule, our example  was: During take-off and landing, ... 'you have  

to' was the best choice: external obligation.  Also possible to say: You must fasten your  

seat belt. Don't say 'should' because that just  implies that there's no rule: do it if you like,  

but it's advisable. I just wanted to make sure  that the difference, the feeling difference,  

tone difference, between 'must' and 'should' is  clear. It's also different for the second use  

of 'must'. For example, we saw that 'must' is for  evidence-based assumption. So, you have evidence,  

you have a piece of evidence, that tells you this  must be true or not true. 'should' expresses your  

expectation. It doesn't ... It's not based on  a piece of evidence. So, for example, we had  

the example: Jill's car is not outside. She must  have gone shopping. That is an assumption. But,  

if you think, okay, so now she's been out for  two hours, she should be back soon. That's what  

you expect to happen. Don't say: She must be  back soon. Here, ,should, is the only word that  

works. And someone may say: Oh, but she's been  gone for a couple of hours, so I expect her to  

have come back by now. Again the word to use here  is 'should' not 'must': She should have been back  

by now. That's what I expect to have happened.  So lots of information! You may need, again like  

every time with modal verbs, to review this and  or use the summary that I will share with you.  

Let's do a quick summary here to end.  So, to express necessity or obligation,  

the difference is minimal and negligible,  necessity/obligation to do something positive:  

must, have to, need to. Necessity or obligation  not to do something: only 'must' works: must not,  

in the negative. To remove the necessity,  no necessity to do something only 'have  

to' and 'need to' work in the negative: not  have to, not need to. Coming to assumption:  

Assuming that something is true: 'must',  but assuming that something is not true,  

'must' doesn't work: it's 'cannot' and 'could  not'. And, the difference with 'should':  

'should' is advice to do or not to do something:  should, should not. And, it's also to express  

your own expectation of things happening or having  happened: should or should not. Right so, that's  

the end of this lesson. I hope it has clarified  the difference between 'must' and 'have to' and  

how to use 'must' correctly in your everyday  exchanges. I hope you've enjoyed this lesson.  

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