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I was wondering if "could" and "should" can be ambiguous in these contexts:

  • I could have done it - meaning either "I had the chance to do it but I didn't" or "I might have done it but I don't remember if I really have" (there is a possibility that I did it)

  • I should have thrown it away - meaning either "I needed to have thrown it away but I didn't" or "I must have thrown it away but I don't remember doing it"

I'm however unsure if "could/should" can be used in such a way.

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  • should have doesn't imply necessity (must have).
    – TimR
    Dec 7, 2017 at 16:07

2 Answers 2

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could have done -- there was a possibility to do

should have done -- there was some call to do

Being unable to remember whether the thing was done is a red herring.

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  • So in case of forgeting we only use "must have"? Dec 7, 2017 at 14:50
  • I'm not sure I entirely agree. I think "I could have thrown it away" is a possibility for the sense of "I might have thrown it away but I'm not sure", although it wouldn't be my first choice for how to phrase it.
    – stangdon
    Dec 7, 2017 at 14:53
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    When the context is your foggy memory, "there was a possibility to throw it away" can be understood to mean "I might well have thrown it away". But the meaning of the phrase is still "there was (certainly) a possibility to throw it away" or "It is not impossible that I threw it away". I should have thrown it away. I normally do what I'm called upon to do, so I might well have thrown it away.
    – TimR
    Dec 7, 2017 at 14:56
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I could have done it means that I had the means and opportunity to do it, but I am not confirming whether or not I actually did it. It doesn't really have an "ambiguous meaning," but it is consistent with a number of real-world situations:

  1. I had the opportunity to do it, but didn't actually do it.
  2. I had the opportunity, but I don't really remember or know whether I did it.
  3. I did do it, but don't want to tell you that I was the one who did it.

Again, that's not an "ambiguity" per se, it's just that there's a limit to the amount of information conveyed by the sentence I could have done it - it's just a statement of my having had the ability to do an action in the past. It's natural that there are multiple real possibilities once you speculate beyond the limits of what the sentence says.

For your other sentence: I should have thrown it away means that I had the means and opportunity to throw it away, I had some obligation or need to do so, but I have a positive belief that I failed to throw it away. It could be the case that my belief is mistaken, but I would not use this sentence if I could not remember whether I threw it away or not.

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  • I don't believe that should have implies that something didn't happen. I washed my hands because I should have. See Tᴚoɯɐuo's answer.
    – Davo
    Dec 7, 2017 at 18:04

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