Modality and modal auxiliary verbs: the essentials

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

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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.

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Confusing modal verb 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
  Probability / Likelihood
      He will do that
      I expect she'll be late
  Certainty
      That must be the right answer
      That's definitely wrong
  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 plain modals) and come in three flavours:
  • pure 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 central modal 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 verbs although on this site that term is reserved for the next group.
  • marginal modal verbs are verbs which express some form of modality but do not share the structural characteristics of pure or semi-modal 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 and adjectives 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
  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.
• modal nouns are also sometimes included in the list.  These are nouns often formed from modal adjectives and include, for example:
      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 so on.

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It might be a unicorn As you can see from the above, modality needn't be expressed using modal 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 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.

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Function

It's not always as simple as it seems.  English uses modal 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.

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 verbs.  If you don't do this, you will do more harm than good by confusing your learners.

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Form

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

can | could | may | might | shall | should | will |would | must | ought to

Semi-modal verbs

used to | need | dare

Click here when you've done that.

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Some common modal verbs explained

This is a short and partial overview of the functions of some of the common modal 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
or
    He can't be there yet.

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
   or
       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)

must

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.

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 pure modal verbs and hence, the verb is considered here.

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Time and tense

Only some modal 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

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Aspect

Usually, when a modal 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 verbs.

Here's a pictorial (and incomplete) summary of the pure modal verbs.  Your task is to look through it and make sure you can devise an example of all the functions.

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Click here to do a short test.

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Related guides
grid of the common modal verbs	for reference
PDF document	a short PDF document covering some of the above
tense and aspect	for some more information and the distinction
modality	for the in-service guides to modality and modal verbs
modal auxiliary verbs one by one	for a traditional guide to pure (or central) modal auxiliary verbs
semi-modal verbs	the guide to semi- and marginal modal verbs

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