Is there any difference between the following sentences?

If Jones was at work until six, he can't have done the murder.

If Jones was at work until six, he couldn't have done the murder.

  • The first might be said by a detective in the course of an investigation. The second might be part of a story, describing the detective's reasoning in the past tense. Oct 10, 2020 at 15:05
  • Why are people silently downvoting this question? Oct 10, 2020 at 15:06
  • @FumbleFingersReinstateMonica I Dved. I DVed simply that considering the rep amount the OP has, I wondered why she/he couldn't have distinguished such (probably) easy 2 sentences. Sorry for having not said prior to DV.
    – user17814
    Oct 10, 2020 at 15:15
  • @Kentaro: You seriously think this is an easy question? I think it's a tricky issue even for native speakers! Oct 10, 2020 at 15:27
  • @FumbleFingersReinstateMonica The second sentence is outright past tense. If you don't like my "judge", your "retaliation" is very welcome.
    – user17814
    Oct 10, 2020 at 15:35

1 Answer 1


1: If Jones was at work until six, he can't have done the murder.
it is not possible (now) that he did it (then)
2: If Jones was at work until six, he couldn't have done the murder.
it was not possible (then) for him to do / have done it (then)

Where in practice both assertions can only actually mean the same thing. If he couldn't have done the murder at the time the murder was committed, it remains true forever into the future that he can not have done it.

Which tense to use is a stylistic choice in contexts like this - it depends on whether you want to focus attention on the current situation (he IS innocent), or on what happened in the past (he DIDN'T do it).

EDIT I found this in commonenglisherrors.com...

While Can’t Have (Been) is used to refer to an incident in the near past, Couldn’t Have (Been) is used for an event that happened way back in the past.

...which arguably is simply another way of describing the "attention focus" point I made above - since can't focuses on the Present, and couldn't focuses on the Past, it's perfectly natural to see the latter as more appropriate when that "Past" is a long way away from the "Present" (it's in the more distant past).

  • To make it clear, it is correct to say "cannot have v3" is past form of the ability, while " could have v3" is past form of the possibility from grammar perspective.
    – Mrt
    Oct 10, 2020 at 15:10
  • But "can" is also used for "possibility" in the negative form in the present such as " it can't be true."
    – Mrt
    Oct 10, 2020 at 15:11
  • I don't understand the point you're making. But I have come across a claim that there can be a difference, so I'll edit that in. Oct 10, 2020 at 15:19
  • ...but I can say that your "ability / possibility" distinction is not relevant here. Oct 10, 2020 at 15:32

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