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Modal verbs of obligation

Modal verbs of obligation

Modal verbs part 2 - obligation :
must, have to, should and ought to , need to

NOTE: The term "modal verbs" can be used in different ways. Used as a syntaxic category, it only refers to the single-word verbs can, could, may, might, must, shall, should, will, would. Used as a semantic category, it includes the single word modals plus other verbs which  express modality in the same way. For students and learners, modality is easier to understand when treated as a semantic reference.
   The failure to distinguish between the semantic and syntaxic uses of the word "modal" is the main reason for the enormous confusion that exists around the question of "what constitutes a modal verb?" For a discussion of this see  Modal verbs, why the confusion?
There are two types of  modal verbs of obligation ;
  • those that primarily express a firm obligation or necessity - must and have to 
  • those that express a recommendation or moral obligation - should and ought to , and need to

Firm obligation, etc. - must and have to  (and got to)

The verb must only exists in the simple present and present perfect forms.
While the present form can express obligation, necessity, certainty or strong probability, the present perfect forms only express a strongly felt opinion or supposition.
All persons Present Present perfect
Affirmative must
must have
Negative must not, mustn't  must not have,  mustn't have
If other tenses are required, the speaker or writer must use forms of the synonymous modal verb "have to" .  This modal auxiliary has all normal tenses, including progressive or continuous forms; these are not common, but need to be used in some cases.
Principal tenses Present Present perfect Past Future
Affirmative: has to,
have to
has had to
have had to
had to
will have to

does not have to,
do not have to ,
doesn't have to
don't have to
has not had to
have not had to
did not have to
didn't have to *
will not have to
won't have to
Progressive or continuous am having to
is having to,
are having to
has been having to
have been having to
was having to
were having to
will be having to

* The form "had not to" is sometimes used, but it is generally considered to be archaic.

Examples of must and have to being used to express :
a. Firm obligation or necessity
b. Certainty  or strong probability.
c. Must have only :  supposition  

a1) You must see a doctor at once !
a2)  I have to be at school tomorrow at 8 a.m. I have an exam !
a21) I've got to be at school tomorrow at .....
a3)  You mustn't touch that plate, it's too hot.
a4)  I had to see a doctor, because I felt very sick.
a5)  I had to break the window ! I lost my key !
a6)  The manager isn't here, he's had to go to Washington on urgent business.
      For more examples, check out the e-book or paperback, available from Amazon

b1)  He must be over eighty, he was born in 1930.
b2)  If my brother's not in London, he has to be in New York.

c1)  I can't find my laptop, I must have left it in the train.
c2)  If they're out, they mustn't have heard the news.
Take care !  
Take care to distinguish correctly between  "had to" and "must have"
They had to go to London
   = They were obliged to go to London
They must have gone to Chicago  
   = In my opinion, they have certainly gone to Chicago.

Got to :

In informal styles, most commonly in the present tensehave to is often accompanied  by the word got, particularly in spoken English.
     For example an alternative to I have to is I've got to.
More examples:
He's broken my computer, so he's got to get me a new one.
We're late, we've got to hurry.
You've got to work harder if you want to get top marks.
Before got, the auxiliary have is almost always contracted (i.e. They've got, not they have got).
For more on got, see Get and got

► Other uses of the verb have: see the verb to have.

Must and have to in negative contexts

It is vital to remember that must not and don't have to are NOT synonyms – indeed they mean two completely different things.

Must not implies negative obligation (i.e. being forbidden to do something)
Don't have to (like haven't got to) implies the absence of obligation (i.e. having no obligation to do something)
They must not go to London
   = They must stay away from London,  they cannot go there.
They do not have to go to Chicago.  
   = They are not obliged to go to Chicago, but they can go there if they wish.

Recommendation or moral obligation - should and ought to,  need to 

a) should, ought to

The verb should only exists in the simple present, and present perfect forms
  Forms of should
All persons Present Present perfect
Affirmative should
should have
Negative should not, shouldn't  should not have,  shouldn't have

The verb ought to only exists in simple present and present perfect forms
  Forms of ought to

All persons Present Present perfect
Affirmative ought to
ought to have
Negative ought not to,  oughtn't to ought not to have,  oughtn't to have

Should and ought to are more or less synonymous

a1) You should stop smoking ( = You ought to stop smoking.)
a2)  It's raining hard, the children ought to come indoors.
a3)  I didn't know you were married ! You ought to have told me !
a4)  If you'd wanted to succeed, you should have worked harder at school.
a5)  This pullover's got holes in it, I should get a new one.
a6)  This pullover's got holes in it, I ought to get a new one.
a7)  That's awful ! You really oughtn't to have done that, you know !

b) Need to :

Used affirmatively, need to implies strong recommendation; but used negatively it expresses an absence of obligation. There are two negative forms of need, either don’t need to or needn’t. 

Take care!   Needn’t is never followed by to. Need is also used as a main verb followed directly by an object, as in I need you.

b1) I’ve been working non-stop for six hours, I need to take a rest.
b2) I think you’ve got covid….. you need to get tested at once.
b3) Thank goodness, I haven’t got covid, so I don’t need to stay at home
b4) Thank goodness, I haven’t got covid, so I needn’t stay at home.

More modals:  Modals of ability - can, may etc.
►  See also: use of to be as a modal verb

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Les verbes modaux d'obligation

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