Modal verbs of obligation

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

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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
Negative;	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

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* 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.
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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 tense, have 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)
Compare:

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

Examples:
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.

Examples:
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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