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    #1

    can have done

    Longman Grammar And Vocabulary For Cambridge Advanced And Proficiency Side-Wellman says:
    Past
    We use could have, will have, may have and might have
    to speculate about the possibility of something
    happening in the past (see Unit 4, Section 2 for
    modals in the past):
    They muy have finished already, for all 1 know.
    The doctor won't have had a chance to look at your
    X-ray yet.
    Sometimes we speculate about something that
    didn't happen but we feel there was potential for it
    to happen:
    That wasn't a good idea - you might have hurt him.
    Sometimes we speculate about what happened
    without knowing exactly what did happen:
    You muy / might / could have done just enough to
    snape through.
    My question is: Can we use can have done instead of those to mean the same thing? If we can, would you please show me some examples?
    Thank you very much.
    Last edited by norwolf; 16-Dec-2008 at 10:02.

  1. Raymott's Avatar
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    #2

    Re: can have done

    Quote Originally Posted by norwolf View Post
    Longman Grammar And Vocabulary For Cambridge Advanced And Proficiency Side-Wellman says:
    Past
    We use could have, will have, may have and might have
    to speculate about the possibility of something
    happening in the past (see Unit 4, Section 2 for
    modals in the past):
    They may have finished already, for all 1 know.
    The doctor won't have had a chance to look at your
    X-ray yet.
    Sometimes we speculate about something that
    didn't happen but we feel there was potential for it
    to happen:
    That wasn't a good idea - you might have hurt him.
    Sometimes we speculate about what happened
    without knowing exactly what did happen:
    You may / might / could have done just enough to
    snape through.
    My question is: Can we use can have done instead of those to mean the same thing? If we can, would you please show me some examples?
    Thank you very much.
    No, you can't say that. "could" is used as a past tense of "can" and to indicate possibility or, as you say, speculation. So you'd say "could have done".

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    #3

    Re: can have done

    Quote Originally Posted by Raymott View Post
    No, you can't say that. "could" is used as a past tense of "can" and to indicate possibility or, as you say, speculation. So you'd say "could have done".
    Thank you, Raymott.
    This sentence is by Alexander L. G:/ wonder where he can have left the key.
    But we can't say "He can have left the key last night",right?



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    #4

    Re: can have done

    I’m not a teacher.

    There is your question: Can we use can have done instead of those to mean the same thing? If we can, would you please show me some examples?

    Hi Norwolf,

    I know the following usage of could + perfect infinitive

    1. He could have given an answer yesterday but he refused to.

    2. They could have helped him.

    3. “Oh! “ cried Fleur. “You could not have done it.”

    4. Could they have completed the work? They had so little time.

    But I know also the following passage from my English grammar:

    When incredulity, doubt, or astonishment is expressed, the time of the action is indicated by the form of the infinitive and not by the form of the modal verb, as both can and could may refer to the present and to the past.

    If the action refers to the past, the perfect infinitive is used.

    5. Can he have failed us?

    6. Peter can’t have translated this article, he does not know Spanish.

    7. Can (could) she have said that?

    8. Can she have been waiting for us all the time?

    9. Can I have my photo taken?

    10. What could she have seen in that fellow Bosinney to send her mad?

    In the last sentence (Nr.10) we can easily replace could by can without any change of meaning. Thus, when expressing incredulity could loses its temporal meaning. Could with the perfect infinitive has almost the same meaning, only the negation is not so catgoric as with can.

    11. What can she have seen in that fellow Bosinney to send her mad?

    12. They saw us. I hope they didn’t recognize us.
    The car was going too fast. They couldn’t have seen us.

    In this meaning could is mostly used with the perfect infinitive. Other forms of the infinitive are hardly used.

    13. Can it be that she has no read this book?

    Regards,

    V.


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    #5

    Re: can have done

    There is a restriction on the use of 'can' in the meaning of supposition in positive sentences.

    Can it be true?
    Can it have been true?
    It can't be true.
    It can't have been true.

    But: It may / might / could be true.
    Or: It may / might / could have been true.

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    #6

    Re: can have done

    Quote Originally Posted by norwolf View Post
    Thank you, Raymott.
    This sentence is by Alexander L. G:/ wonder where he can have left the key.
    But we can't say "He can have left the key last night",right?

    right

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