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Concourse 2

Modality and modal auxiliary verbs: the essentials


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:


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.



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).

think For example, what do these sentences imply about the perceptions of the speaker?
Click here when you know.
  1. He may choose a book.
  2. He can open the door.
  3. He should be in London.

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.



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

Pure or central modal auxiliary verbs
can | could | may | might | shall | should | will |would | must | ought to
Semi-modal auxiliary verbs
used to | need | dare

Click here when you've done that.


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:

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

may / might

This verb can express:

  1. Permission:
        You may ask a question now
        May I smoke here?
    (More rarely and more formally: Might I speak to you?)
  2. 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.

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

Should is much more common and can express:

  1. Obligation:
        You should write to your mother more often
  2. Logical deduction:
        He should be there by now
  3. 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
    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

  1. 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
        I would be happy to help.)
  2. 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
  3. 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
  4. Insistence (present and past):
        He will keep arguing with me
        He would keep changing the subject
  5. Probability:
        That will be him at the door now, That would be typical of him
  6. 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.


  1. Obligation:
        You mustn't speak to me like that
        You must be home at 6
  2. 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.

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

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

  1. Obligation (weaker than must):
        She ought to ask if she doesn't know.
  2. 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
    I might ask questions
The first means
    I was able to ask questions
    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

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



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:


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


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


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.


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.


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

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