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Dynamic modality: expressing ability and willingness


The word dynamic derives from the Greek dynamikos meaning strong or powerful.
Dynamic modality is one of the usual four categories of modality (the others being epistemic, deontic and alethic to which there are separate guides).
If these terms are mysterious to you, you should follow the guide to types of modality before tackling what follows.
There is, in fact, an argument that dynamic modality is not actually a form of modality at all because the speaker / writer is not:

  1. using a modal expression to express an opinion regarding the truthfulness or necessity of an event, state or action
  2. affecting the current situation by what is said

For example, with the modal auxiliary verb can, four clear types of modality can be expressed:

Example Type of modality expressed
You can go now Deontic: giving permission / removing the obligation to stay
It can't be a rectangle if it has five sides Alethic: expressing a factual state (also, arguably, not really a modal use at all)
That can't be the right answer Epistemic: expressing the speaker's view of the truth of a proposition
She can play the piano very well Dynamic: stating a simple fact about ability

Traditionally, following Palmer (1986) only the modal auxiliary verbs can and will express dynamic modality, i.e., express ability or willingness.
There are other ways to express the same notions, of course, and the grammar that is involved is not as straightforward as it might be.
The following is an analysis which is focused on teaching the concepts and notions.


Expressing willingness

Both will and can are used to express the notion of willingness as in, for example:

  1. Jane will drive you to the station if you like
  2. Jane can drive you to the station if you like

Functionally, these two utterances are identical: they both express the subject's (Jane's) willingness to do something and do not express the speaker's view of any existing obligation or its lack or any comment concerning the truthfulness of a proposition.
There is, however, a difference in sense:
Sentence 1. concerns willingness alone but sentence 2. may be construed to suggest that not only is Jane willing to drive you to the station but that she is also able to do so (because, e.g., she has no other outstanding commitments).
The differences become clearer when set in a dialogue:

Fred: Jane will drive you to the station if you like Fred is stating that he understands that Jane is willing to do this
Jane: Sure, no problem Jane is confirming her willingness
Jane: Sorry, I won't drive in the dark Jane is stating her unwillingness
Fred: Jane can drive you to the station if you like Fred is stating that he understands that Jane is willing to do this
Jane: Sure, no problem Jane is confirming her willingness
Jane: Sorry, I've got to take the kids to school Now she is stating her inability rather than her unwillingness

The border between the notions of willingness and ability is blurred in English because we can express both functions with the same modal auxiliary, can, and with no change to the intonation.
An example of this is a statement such as:
    I can help you with your homework
in which it is unclear (without a very explicit context) whether the speaker is stating an ability or a willingness to do something.
The same ambiguity occurs with the interrogative form:
    Can you help me with my homework?
in which the questioner may be referring to the hearer's ability or willingness.
With the negative form, this ambiguity disappears because:
    I can't help you with your homework
implies inability rather than unwillingness.
In order to state unwillingness, we have to use the modal auxiliary will:
    I won't help you with your homework


will / would: the multi-faced verb

Much confusion can be avoided if the verb will is considered polysemous (i.e., having two or more meanings).  In English the verb can, among other things, express

  1. Futurity (a non-modal use) as in, e.g.:
    1. I'll be 50 tomorrow
    2. The train will arrive at platform 6
  2. Certainty (epistemic modality) as in, e.g.:
    1. That'll be the postman
    2. This'll be a long job
  3. Willingness (a dynamic modal use) as in, e.g.:
    1. I'll help you
    2. She'll read you a story

We are only concerned with the last of these here.  These meanings are quite radically different and mixing up examples of the three functions in the same classroom presentation can bewilder learners unnecessarily.

The situation is similar with the verb in the past, would.  It can, among other functions, signify:

  1. Distancing and politeness
    1. Choosing
          Would you help?
          Will you help?
      or choosing
          If I give you the money will you buy it for me?
          If I give you the money would you buy it for me?
      implies greater politeness and tentativeness although both refer to current willingness
  2. Indirect speech
    1.     I'll do that for you
      is reported (at another place and time) as
          She said she would do it for me
      but both refer to her willingness
  3. The habitual aspect
  4. He would often complain
    expresses a habitual or repeated action and is not to do with (un)willingness at all


Expressing ability

The verb can is often the first modal auxiliary verb, along with must, that learners encounter in English.  It expresses a simple and fundamental concept and is, in the present tense at least, uncomplicated so we have, simply:

  • She can do that
  • She can't do that
  • Can you do that?
  • Can't you do that?

In the present tense, all these instances of can are replaceable with a copular verb (usually be) and the adjective able plus the preposition to:

  • She is able to do that
  • She isn't able to do that
  • Are you able to do that?
  • Aren't you able to do that?

although the use of the able-to formulation is less common and more formal.  When no difference in meaning exists, English speakers will usually prefer the modal auxiliary.
Other copular verbs make subtle changes to the meaning but the sense remains:

  • She seems able to do that
  • She doesn't appear able to do that
  • She ended up able to do that
  • Did she prove able to do that?

That this formulation is analysable, and teachable, as copular verb phrase + adjective phrase + to + verb phrase is revealed by the parallel forms such as:

  • I am happy to do that (expressing willingness)
  • She is reluctant to do that (expressing unwillingness)
  • She is wholly unable to swim (expressing inability)
  • Isn't she required to be here (expressing a lack of obligation)

and so on.


Expressing ability in the past

In the past tense, however, things become a little more difficult.


General and specific abilities

  1. A general ability in the past can be expressed either with could or was / were able to:
        I could speak French when I was 10 = I was able to speak French when I was 10
        When I worked in London, I could catch the train to work = When I worked in London, I was able to catch the train to work
  2. However, if we refer to a specific instance of success, only was / were able to is possible:
        He was able to say the word in French
        *He could say the word in French
    Other specific instances of an ability can be expressed with other formulations, often verb processes such as:
        We succeeded in getting tickets
        They managed to catch the train
    But not, usually:
        *We could get tickets
        *They could catch the train

Three exceptions

  1. Verbs of perception can be expressed as abilities in the past with no sense of whether it is a general or specific ability to which we refer.  E.g.:
        She was able to hear the telephone ringing = She could hear the phone ringing
        He was able to see the harbour = He could see the harbour
        I couldn't understand him = I wasn't able to understand him
  2. With negative-sense adverbials.  E.g.:
         I was hardly able to hide my feeling = I could hardly hide my feelings
         They were only able to find expensive tickets = They could only find expensive tickets
  3. With subordinate clauses.  E.g.:
        They were delighted she could come = They were delighted she was able to come
        He said he couldn't come before 6 = He said he wasn't able to come before 6

Fulfilled and unfulfilled ability

In the past, the perfect form of could implies an unfulfilled ability but the was / were able to formulation implies a fulfilled ability.  For example:

  • I could have spoken to her on the phone
    refers to the fact that I was able to do so but didn't for some reason
  • I was able to speak to her on the phone
    refers to the ability and to the fact that I did speak to her
  • Could they have talked to her? / Couldn't they have talked to her?
    both refer to whether they had the opportunity and the ability but assumes that they did not talk to her

In the negative, could have usually refers to possibility, rarely to ability, so it is not easily usable for dynamic modality.  The statement:

  • They couldn't have talked to her
    can only express epistemic modality (i.e., that it was impossible)
  • but:
    I couldn't have driven any faster

    does express ability and is often followed by something like (even) if I'd tried



Because can and could are used for a variety of function apart from dynamic modality concerning ability, there is often no easy way to disambiguate.  For example:

Example Possible modality
You can talk to him Deontic: giving permission to talk to him
Dynamic: expressing ability assumed in another
She could explain clearly Dynamic: expressing her ability in the past
Epistemic: expressing a future likelihood
Deontic: expressing the fact that she should explain
You can't be serious Dynamic: expressing frustration at another's inability
Epistemic: expressing the speaker's view of the truth of a proposition
It could bend Dynamic: stating a fact about the material
Epistemic: stating a possible outcome
Can you be quiet? Dynamic: Are you able to be quiet?
Deontic: Please be quiet!
She could have driven faster Dynamic: she had the ability to drive faster
Deontic: she should have driven faster
You could have told me Dynamic: you had the ability to tell me
Deontic: you should have told me
Epistemic: you may have told me but I don't remember

Only the context and understanding of speaker intentions along with some knowledge of the shared information can determine which meaning is appropriate.

Related guides:
non-modal-verb modality covers some of the above and extends it to include epistemic and other forms of modality
epistemic modality which follows a similar format to this one
deontic modality which follows a similar format to this one
the modality map for more choices

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