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If "can" can't be used in the past, so why the rule in the book named "New Round Up 5 students book" says the following: can't/couldn't + perfect infinitive = it's impossible that something happened in the past. Example: She can't/couldn't lost her way (=it's not possible she lost he way.). So, according to the rule "can't" can be used with past participle. I am confused. pleas help

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  • What your book is saying is that epistemic "can" can be used with a past participle, but it is restricted to non-affirmative contexts, e.g. the perfect infinitival "She can't have lost her way."
    – BillJ
    Nov 19, 2017 at 10:10
  • Well, so much at least is now clear,' said Legolas: 'Frodo is no longer on this side of the River: only he can have taken the boat. But then Legolas was not a native speaker. Nov 19, 2017 at 11:35
  • On the other hand, there can have been no precise definition of military service in Normandy before the mass of the Norman fighting-men had accommodated itself to French methods of warfare, and the most ancient Norman families were settled in their possessions before this development can have taken place. Frank M. Stenton, Anglo-Saxon England. Oxford. 3rd edition. 1971. Nov 19, 2017 at 11:42
  • @BillJ: The OP can forget the confusing rule he opens with, that "can can't be used in the past (but can't can)." Nov 19, 2017 at 14:59
  • @Tᴚoɯɐuo Why? How do those examples help the OP, who is most likely a non-native speaker looking for a clear straightforward answer, not what you consider to be exceptions? In any case, your two examples have a non-affirmative implication: in the first, "only" is semantically close to a negative, and in the second, "can" is in a negative clause. The OP would do well to remember that epistemic "can" is only used in non-affirmative contexts.
    – BillJ
    Nov 19, 2017 at 15:01

1 Answer 1

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Formally, most modals have a present and a past form, e.g. Can/could, will/would, shall/should, may/might.

I have no idea what you mean by a perfect infinitive.

"She can't have lost it" means it was not possible for her to lose it at any time in the past up to the present.

"She couldn't have lost it" means exactly the same thing.

Why does the tense of the modal not make any difference?

Because "lost" conveys the timing of the event on its own. So the tense of the modal is irrelevant.

Personally, I think it is better style to make the tense of the modal and main verb agree in this case, but millions have a different opinion.

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