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I know there are some tenses where we mustn't use "can", instead of that we must use "be able to" like these sentences:

  • They won't be able to play
  • I haven't been able to sell it

Are there more cases where we cannot use "can"? Which are all the verb tenses where we cannot use it?

Thanks.

  • I think you should change this question and ask something way more specific or interesting; a list of those words will never be exhaustive. – M.A.R. Sep 3 '15 at 18:24
  • I voted to reopen this question, not because I don't think it's too broad (I think it's a little too broad when we want to exhaust all possible cases), but because a) I believe that we can have a summary of useful and common cases, which would be useful for most learners, and b) it looks like we already have one such answer. – Damkerng T. Sep 4 '15 at 4:54
  • Also related (for be able to on a specific occasion in the past): 'managed to' vs. 'could'. – Damkerng T. Sep 4 '15 at 5:02
  • OK, I can make one more complete question – ChesuCR Sep 4 '15 at 19:12
5

(I assume you mean tense in the ordinary “layman’s” sense which includes all the verb constructions rather than one of the narrower technical senses in which the word is used by linguists.)

Can, like all the other English full modals, is “defective”: it is employed only in two finite forms, and has no infinitive or participial forms. Consequently, it can only appear as the first verb in a construction ( marks unacceptable utterances):

  • It cannot appear as an infinitive following another modal or in an infinitival clause:

    I will can do that. → okI will be able to do that.
    I want to can do that. → okI want to be able to do that.

  • It cannot appear as a past participle following HAVE in a perfect construction:

    I have canned do that. → okI have been able to do that.

  • It cannot appear as a gerund-participle:

    Canning do so, I left. → okBeing able to do so, I left.
    Canning do that takes study. → okBeing able to do that takes study.

    (It also cannot appear following BE in the progressive construction, but neither can be able to, ordinarily, because the sense of both is stative:

    I am canning do that. → I am being able to do that.

  • And of course no modal verb or semi-modal verb construction can appear in the passive, because these are all intransitive:

    I was canned. → I was been able.

Can has one quirk which it does not share with the other full modals: the modal perfect can have done ordinarily appears only in the negative, or (very rarely) as an emphatic contradicting a preceding negative:

He can have finished already. but
okHe cannot have finished already. and
okNot so: he can have finished already, and has done so!

  • +1 Nice post - very helpful for learners. But.. Hmmm, how about: "The activity can have already commenced when the application is made, or it can be made prior to the company conducting the activity outside Australia." or some examples on these pages here? – Araucaria - Not here any more. Sep 3 '15 at 12:44
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    @Araucaria Anything can happen--or can have happened. But it's very rare: may is about 500 times more likely there. Note, too, that it's almost impossible in the core (non-modal) sense replaceable by is able to. ... But, yeah: this is why I gave up on writing a Canonical Post on modals! – StoneyB on hiatus Sep 3 '15 at 13:42
  • Although the question asks mainly about can, the OP (and the future readers) may also want to read about could and managed to, and may find this related post useful. – Damkerng T. Sep 4 '15 at 5:00
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The modal verbs can only be used in present tense and in past tense. They have no infinitive and no past participle as normal verbs. So you can form no will-future or a conditional form with would. And you can't form perfect tenses.

"must" is normally used only in present tense. The use of must (past tense) is restricted to special cases.

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