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What is the difference between "couldn't come" and "couldn't have come"?

When we compare the below two example sentences:

She couldn't come to the party.

and

She couldn't have come to the party.

Is there any difference in their nuance and meaning?

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The two could's do not mean the same thing.

In She couldn't come, the could is an alethic modal, which refers to ability.
So it means 'She was not able to come', which entails that she did not come.

In She couldn't have come, however, the could is an epistemic modal, which refers to possibility.
This is Logical possibility, which is based on evidence and logical conclusions from it.
Consequently, it means 'It was not possible that she came', which entails that she did not come.

Note that both readings have the same entailment; this is not uncommon.
Every modal auxiliary verb has at least two readings;

  • one reading is usually epistemic
    This may not be the place. This must be the place. This should be correct.
  • the other reading is either deontic
    If he will agree, it's a deal. She must see him at 4pm. We should wait in that room.
  • or (in the case of can and could) alethic
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    Alethic modality is possibility/necessity; epistemic modality is belief/knowledge (cf. "epistemology"). Linguists are in the habit of misusing "epistemic", but I think you've added to the confusion. – Greg Lee Jan 4 '16 at 20:56
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"She couldn't come" indicates that she was unable to come. For example, "She couldn't come to John's birthday party last week".

In contrast, "She couldn't have come" suggests that it's impossible for her to have come, usually because of some conflicting reason. For example, "She couldn't have come to John's birthday party last week, because she called me that morning to tell me her flight home was canceled". This construction is also often used to contradict someone. For example: "— Mary tells me Jane came to my birthday party? I didn't see her. — She couldn't have come, because she was sick that morning."

  • so "could'nt come" is a fact and "couldn't have come" is a conclusion? – Daniel Xhemalce Jan 4 '16 at 17:18
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    @Daniel Xhemalce: You couldn't really draw that inference, no. But the usual reason for using past perfect is to indicate some connection to the present (time of utterance difference), so it will often turn out that the reason you're confidently saying She couldn't have come... (in the past) is that you're going to continue with ...because [of some fact that you now know]. – FumbleFingers Reinstate Monica Jan 4 '16 at 17:46
  • But "If she couldn't have come to my party, I wouldn't have invited her." – Greg Lee Jan 4 '16 at 20:50

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