Modality and modal auxiliary verbs: the essentials

modals

What follows refers only to English. Languages deal with the modality in a bewildering variety of ways.

What is modality?

Modality differs from tense and aspect in that it does not refer directly to any characteristic of the event, but simply to the status of the proposition.
(Palmer, 2001:1)

In other words, modality concerns how the speaker / writer perceives a state of affairs.

Confusing modal terminology

Terminology in this area can get a bit confusing because authorities differ about the names we give to things. Here's a short glossary:

modality
refers generally to the way speakers express their view of an event in terms of possibility, ability, probability, likelihood, certainty, permission or prohibition. For example, I can say:
Permission / Prohibition
    He must not do that
    He is not allowed to do that
    He is not supposed to do that
    That is forbidden him
    I forbade him to do that
Possibility / Ability
    He might do that
    He isn't able to do that
    She has the strength of mind to succeed
Probability / Likelihood
    He will do that
    I expect she'll be late
    I doubt she's coming
Certainty
    That must be the right answer
    That's definitely wrong
    That's obviously not true
and these are all examples of modality in one form or another expressing the speaker's view of the status of the event.
modal auxiliary verbs are often called modal auxiliaries (or even, sloppily, just modals) and come in three flavours:
- central modal auxiliary verbs are:
  could, might, may, should, would, must, can, will/shall, had better, ought to
  but even here you will find verbs included in or excluded from this list. Some will call these pure modal auxiliary verbs.
- semi-modal auxiliary verbs are usually listed as:
  need, have to, dare, used to
  although some will exclude have to from that list and some will include ought to and be able to in the list. These are sometimes called marginal modal auxiliary verbs although on this site that term is reserved for the next group.
- marginal modal auxiliary verbs are verbs which express some form of modality but do not share the structural characteristics of pure or semi-modal auxiliary verbs and there are lots of them including, e.g.:
  be supposed to, seem to, be likely to, be about to, tend to etc.
modal adverbials, adjectives, nouns and verbs are often included in the list because they, too, express similar concepts. This area includes, for example
    it is important that
    it is vital that
    it is unlikely that
    it is necessary that
    necessarily
    importantly
    vitally
    crucially
    you are obliged to
    she made me do it
    they let her leave
    I advise you to
    the obligation is on you
    there is no likelihood of it happening
    There is a possibility that it will rain
    I am making the assumption that he's coming
    There's a strong probability that it will rain
and a host more. They are all ways of signalling the speaker's view of an event but do not use special verbs to do it.

It might be a unicorn

As you can see from the above, modality needn't be expressed using modal auxiliary verbs. We can say, for example
It seems unlikely to me that she has seen a unicorn
which expresses our view of the event.
We do use modal auxiliary verbs for this kind of thing quite often, however, as in
She can't have / mightn't have / couldn't have / must've / may've seen a unicorn.

See if you can match modal to function in this little test.

Function

It's not always as simple as it seems. English uses modal auxiliary verbs in a complicated way and the area causes endless problems for learners (and teachers).

For example, what do these sentences imply about the perceptions of the speaker?
Click here when you know.

He may choose a book.
He can open the door.
He should be in London.

Sentence 1 means either
    He is allowed to choose a book
or
    It is possible he will choose a book.
Without a context, it simply isn't possible to know which because the verb expresses obligation or permission as well as possibility.
Sentence 2 means either:
    He is able to open the door
or
    He is allowed to open the door.
Again, with no context, we do not know what is meant because the verb expresses both ability and permission.
Sentence 3 means either
    He is supposed to be in London
or
    He is likely to be in London.
And here the verb expresses either obligation or possibility / likelihood.

So, issue number 1 for teachers of English is always to provide a clear context for modal auxiliary verbs. If you don't do this, you will do more harm than good by confusing your learners. This kind of structure cannot be usefully taught at sentence level because, as we have seen, the message is unclear.

Form

Here's a list of some modal auxiliary verbs in English. What's different about the semi-modal auxiliary verbs?
Think about:

How we make a negative with all of these verbs.
How we make a question with all of these verbs.
Whether we put an -s ending on the third person singular of the verbs.

Pure or central modal auxiliary verbs

Semi-modal auxiliary verbs

used to | need | dare

Click here when you've done that.

For all central modal auxiliary verbs:

Negatives:: we make negative by putting not after the verb so we get
He can go → He can't (cannot) go
not:
*He doesn't can go.
Questions:: we make a question by simply reversing the order of verb and subject so we get
We can come → Can we come?
not
*Do we can come?
-s: We do not put an -s on the third person singular form:
He must
not
*He musts

For semi-modal auxiliary verbs

Here, the situation is more complicated because they sometimes act as 'normal', lexical verbs:

Negatives:: we can make a negative by putting not after the verb
    He needn't go
    He use(d)n't to go
    He daren't go
we can also treat them as normal verbs
    He didn't need to go
    He didn't use(d) to go
    He didn't dare go
Questions:: we can make a question by simply reversing the order of verb and subject
    Need he come?
    Used he to come?
    Dare he come?
we can also treat them normally
    Did he need to come?
    Did he use(d) to go?
    Did he dare go?
-s: we sometimes put an -s on the third person singular form of dare and need
He dares to go
He needs the money)
but never on used to.

Co-occurrence

Pure or central modal auxiliary verbs cannot co-occur. For example, we cannot have:
    *He can must go
    *She will should be here
    *He could must ask
etc.
However, we do allow a central modal to be followed by a semi-modal so, for example:
    He may have to talk to her
    I should be able to get there
are both possible.
The reverse is not the case so a semi-modal auxiliary verb cannot be followed by a central modal auxiliary verb:
    *He has to may talk to her
    *She is able to should be here
are not allowed.

You will be delighted to learn that the complications of semi- or marginal modal auxiliary verbs is not our subject here but you should think for a moment about how the meaning changes depending on the grammar we use. There is a guide linked in the list at the end to semi-modal auxiliary verbs which includes consideration of some marginal modal auxiliary verbs.

Central modal auxiliary verbs are often referred to a pure modal auxiliary verbs. That makes some sense and means the same.

Some common modal auxiliary verbs explained

This is a short and partial overview of the functions of some of the common modal auxiliary verbs.
For more, see the guide to modal auxiliary verbs one by one linked in the list at the end.

can / could

This verb can express:

Ability:
He can read Italian but can't speak it well
He could play the piano well as a child
Permission (present and future):
    Can I come in?
    No, you can't yet
    Could I talk to you tomorrow?
(Im)possibility:
Nobody can be sure
The train could be late

may / might

This verb can express:

Permission:
You may ask a question now
May I smoke here? (More rarely and more formally: Might I speak to you?)
Possibility (present and future):
We may arrive a little late
He might come early

shall / should

Shall is arguably going out of fashion to express the future and there's a strong case to be made for not teaching it for that function, especially at lower levels. However, there are other uses of shall which are still not always replaceable with will. Only the third (and, possibly, the fourth) in this list are common these days and then only in British English.

In questions:
Shall I do my homework now?
Insistence:
You shall do what I tell you
Intention:
I shan't keep you long
Suggestions:
Shall we go?

Should is much more common and can express:

Obligation:
You should write to your mother more often
Logical deduction:
He should be there by now
Advice:
You should take something for that cough

What's the negative of the second example? Right, it's something like
He won't be there yet
or
He can't be there yet.

Both shall and should can act as primary auxiliary verbs forming tenses and that is how they are analysed in the guide to that area, linked below.

will / would

Willingness:
    I'll get the milk
    Will you have another?
(The verb would can also express this sense in
    Would you like another?
but can't be used to express personally willingness except in something like
    I'd like another
or
    I would be happy to help.)
Intentions and promises:
    I'll send you an email soon
    We won't keep you
    He told me he would write soon
    I thought he'd be quick
Prediction (often based on past experience):
    It'll probably rain soon; it often does in November
    He knew it would rain
    He believed I would come
Insistence (present and past):
He will keep arguing with me
He would keep changing the subject
Probability:
That will be him at the door now, That would be typical of him
Characteristic behaviour / habit:
We would often get up really early
When I was younger, I would ... (Compare used to for this function)

Arguably, will / shall / should / would also acts as a primary auxiliary verb forming tenses in English and that is how it is analysed in the guide to primary auxiliary verbs, linked below.

must

Obligation:
You mustn't speak to me like that
You must be home at 6
Logical necessity / deduction:
That must be his father; they are so alike
There must be an error in the data

Negating these meanings is tricky. What's the negative of the following? When you have a brief note of the answers, click for comments.

You must take the medicine every day.
The figure must be correct.

Sentence a. can be negated 4 ways with two different meanings:

You mustn't take the medicine every day (i.e., you are obliged not to)
You needn't / don't have to / are not obliged to take the medicine every day (i.e., there is a lack of any obligation)

Sentence b. can be negated two ways:

The figure mustn't be correct (i.e., you are obliged to give the wrong answer)
The figure can't be correct (i.e., the logical deduction is that it's wrong)

The same considerations apply to forming questions. If we are asking about obligation, we form the question with must as in, e.g.:
    Must they be here tomorrow?
    Mustn't we leave soon?
but when we are are enquiring about what has been deduced or asking for speculation, we use can and could as in, e.g.:
    Can they be sisters, do you think?
    Couldn't it have been the wrong number that you rang?
and so on.

ought to

Obligation (weaker than must):
She ought to ask if she doesn't know.
Logical deduction / expectation:
The bus ought to be here by now.

The question and negative forms of ought, for example:
Ought I to come? (= Would it be best if I came)
She oughtn't to speak to me like that (= It is her duty to be more polite)
are somewhat rare and considered formal.
The forms are, however, parallel to all other central modal auxiliary verbs and hence the verb is considered here.

Time and tense

Only some modal auxiliary verbs have obvious past-tense forms. Here's a list.

Present	Past
can	could
may	could / might
shall	should
will	would
must	(had to)
----	used to
ought to	----
need	----
dare	dared

There are two past forms of may: one for possibility, one for permission. We can say
    I could ask questions
and
    I might ask questions
The first means
    I was able to ask questions
OR
    It is possible I'll ask questions
The second only means
    It is possible that I will ask questions
and cannot be used for ability.

The past of must is often had to but not in all its meanings.
When we are referring to obligations, the past (and the future) are formed with have to as in, for example:
    I had to leave early
    I will have to leave early
etc.
When we are referring to logical deduction, we use the perfect form of the verb to make a past tense as in, for example:
    That must have been his sister
When we make a future tense, we use have to:
The train will have to arrive soon

Aspect

Usually, when a modal auxiliary verb's function is to express ability or permission, we can't use progressive or perfect aspects. For other functions we can use these aspects.
(If any of the above confuses you, try doing the guide to tense and aspect linked in the list at the end when you finish and then coming back to review this page.)

For example:

Possibility

They may have got lost
They may have been driving too long
They can't / couldn't have got lost
They can't have been driving so long

Deduction

I must have left my keys on the table
She must have been working on a solution
You must be joking

Prediction

They will have driven that way
John will still be driving at midnight

This part of the page is available as a PDF document. There is also a reference grid of the common modal auxiliary verbs.

Summary

Here's a pictorial (and incomplete) summary of the central modal auxiliary verbs. Your task is to look through it and make sure you can devise an example of all the functions.
Note that references to futurity are not, strictly speaking, facets of modality.

summary

Click here to do a short test.

Related guides
grid of the common modal auxiliary verbs	a PDF document for reference
PDF document	a short PDF document covering some of the above
tense and aspect	for some more information and the distinction
primary auxiliary verbs	for the guide to a different sort of auxiliary verb which is not modal
modality	for the in-service guides to modality and modal auxiliary verbs
modal auxiliary verbs one by one	for a traditional guide to pure (or central) modal auxiliary verbs
semi-modal auxiliary verbs	the guide to semi- and marginal modal auxiliary verbs

Reference:
Palmer, F. R, 2001, Mood and Modality, 2nd ed., Cambridge: Cambridge University Press