Focus on MUST: an ambiguous modal auxiliary.

difficult of the three modals for foreign students to use correctly. 

MUST is an ambiguous modal auxiliary because it has two different meanings... 

1) MUST/ MUSTN'T : OBLIGATION or INTERDICTION : 

In both cases, the FREEDOM of ACTION is denied.  

- I must go now, or I'll be late. You mustn't prevent me from leaving...

2) but MUST also expresses NEAR-CERTAINTY: I'm almost certain that... 

- He must be sick because he had confirmed that he'd come and hasn't turned up yet.

3) Now what is the sentence in the PAST? 

In the past, when MUST expresses a past OBLIGATION, you must use HAD TO:

 - Mary had to buy tickets because the entrance wasn't free.  

BUT:  'may' also means: => NEAR-CERTAINTY in the PAST:

In this case it will be expressed by: MUST + HAVE + PASTPARTICIPLE

- Mary must have bought tickets, because the entrance wasn't free. (near-certainty in the past)

In such a sentence, only the context will enable you to choose the right modal auxiliary.

4) ) Difference between MUST and HAVE (GOT) TO:

* MUST: the obligation is mostly coming from the speaker or an interlocutor. 

The obligation is coming from 'inside' and expresses a personal decision. 

- Paul must really stop smoking because he keeps coughing... 

* HAVE (GOT) TO : the obligation is coming from other people or other regulations.

This obligation is imposed on people and coming from the outside. 

The speaker may often be quite reluctant to do the action. 

- The doctor told Paul that he really had to stop smoking, but it was difficult. 

 5) Be careful!    In the negative form, its meanings are quite different: 

* MUST NOT: expresses an action which is forbidden: an INTERDICTION. 

- 'No, Pat, you mustn't repeat what I've just told you: it's a secret.'  

* ...DO NOT HAVE TO: you're not obliged to do it. an absence of obligation 

- 'No, you don't have to help her, but you may if you want to.' 

If you think that this lesson is somewhat complex, please, read it over and over again. 

You may also learn a few examples by heart.

Then, you will try the test and will see that everything will be alright! Go for it!  

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1. Sofia must fetchmust have fetchedhad to fetchI don't know her toddler from nursery school because he was sick and couldn't stay.



2. Bernie must be tellingmust tellhas to tellI don't know lies; what he’s saying is unbelievable.



3. 'How long must you waitdid you have to waitmust you have waitedI don't know ?’ ' More than an hour, the line was so long…’



4. Although he had done his best, Max must misunderstandhad to misunderstandmust have misunderstoodI don't know the orders he had received.



5. 'I have no choice, it’s my duty, I had to tellmust have toldmust tellI don't know the truth as a witness.’



6. 'No, you mustn't signdon't have to signmustn't have signedI don't know now, it's not necessary. Your parents will do it for you.’



7. 'I think I have to readmust readmust have readI don't know this book. Frank said it was very good and interesting for my thesis.’



8. He had to believemust have believedmust believeI don't know that he could get away with it, but it was a mistake; he was arrested and judged.



9. 'No! You mustn't readmustn't have readdidn't have to readI don't know her letters! They’re private and can even be intimate…’



10. 'I know it will be quite difficult, but I have to trymust have triedmust tryI don't know if I want to get this promotion.'