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Athabasca University

Modals and Related Expressions

Modals are part of a verb phrase; they give more information about the main verb by qualifying it in some way. Modals also have an effect on the grammar of the verb phrase; after a modal, the infinitive form (verb name) is used. Some modals can be used with different time references, present, past or future; others are restricted to one or two time frames. Some modals can be used in negative expressions, others cannot, and sometimes when used in a negative expression the usage changes. The chart below summarizes the time frames that are possible with the modals and their most common usages.

Modal Past Present Future Usage
be able to * * * Shows ability
I am / was / will be able / unable to do that.
be supposed to * * * Shows expectation
I am / was supposed to / not supposed to do that.**
be to * * * Shows strong expectation
I am / was to be / am not be there by noon / till noon.**
can / could   * * Shows ability or possibility—negative too
I can / can't do that. I could / couldn't do that (tomorrow).
    *   Shows permission or lack of permission (informal, speaking only)
You can / can't have that.
could   *   Request—formal
Could you do that?
  * *   Shows suggestions
You could do / have done that. Couldn't you do / have done that?
  * *   Shows limited certainty—under half
They could be/ have been there.
  * *   Shows impossibility—negative
The keys couldn't be / have been there.
had better   *   Shows advisability
I had better / better not do that.
have got to   *   Shows necessity or lack of necessity
I have got / haven't got to do that.
have to * * * Shows necessity or lack of necessity
I have got/ had / will have to do that.
I don't have/ didn't have / won't have to do that.
may   *   Formal request / permission
May I have that? You may have that.
    * * Shows limited certainty—under half
I may/ may not do that later.**
might   *   Formal request
Might I have that?
    * * Shows limited certainty—under half
I might do that/ I might not do that.**
must   * * Shows strong necessity
I must take the car.**
    * * Shows prohibition
You must not take the car.**
    *   Shows very high certainty
You must be tired after your journey.
ought to   * * Shows advisability
I ought / ought not to do that.**
    * * Shows high certainty
I ought to feel better tomorrow.**
shall   * * Formal question/suggestion
Shall I do that?**
should   * * Shows advisability
I should / should not do that**
    * * Shows high certainty
I should feel better tomorrow.**
used to *     Repeated action in the past
I used / used not to do that regularly.
will   * * Shows certainty / willingness
I'll / wont be there
I'll / won't do it.
    * * Formal request
Will / won't you do that?**
would   * * Formal request
Would you do that?**
    * * Shows preference
I would rather/ would rather not do that.**
  *     Repeated action in the past.
I would do that years ago.
    * * Shows advisability—negative
I wouldn't do that (if I were you).**

** present and future form is the same

Updated September 10 2014 by Student & Academic Services

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