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I was doing grammar exercise, I don't understand why should I use must have been rather than could have been, is it because he suffered from a Long illness and he'll die anyway, so I should put must here? Can someone help me with it I'm kind of confused.

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    Since you can't have an obligation to do something in the past, must have takes its Epistemic sense of logical necessity, rather than its Deontic sense of social obligation. Here it means that (despite his not showing the pain), everyone believed that he was suffering because his situation always caused pain.
    – John Lawler
    May 30, 2020 at 18:12
  • @John: Re "Since you can't have an obligation to do something in the past, must have takes its Epistemic sense of logical necessity...": If the assertion in your introductory clause is indeed true, then why is it that we can use, for example, the periphrastic modal "have to" to express exactly that - an obligation to do something in the past?
    – HeWhoMustBeNamed
    Jun 2, 2020 at 15:27
  • Have to in the past tense does not refer to a (current) obligation to do something in the past, which is impossible, but rather a past obligation to do something. Past (Obliged Do (x)) is not the same as Obliged (Past Do (x)).
    – John Lawler
    Jun 2, 2020 at 15:41
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    @John But the problem I see with your argument that "since you can't have an obligation to do something in the past, must have takes its Epistemic sense of logical necessity, rather than its Deontic sense of social obligation" still remains: If a current obligation to do something in the past is impossible (which of course it is), why can't must have express a past obligation, like have to does -- why can it only take on the Epistemic sense of logical necessity?
    – HeWhoMustBeNamed
    Jun 4, 2020 at 7:58
  • Ah, I see I have misled you. Sorry. That's not the reason why it doesn't. That's just a concomitant arrangement, one of several. There is no "Why" with modals. They are just bloody irregular. After all, they're new (as these things go) -- they used to be real verbs with affixes and tenses and now they're mere skeletons of their former selves. They all refer to future possibility and necessity, either logical or social, and none of them can be inflected (even with the infinitive Zero suffix), and they all hafta be followed by an infinitive. They're still finding their places, and they are weird
    – John Lawler
    Jun 4, 2020 at 16:17

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You are correct, there is nothing grammatically wrong with "could have", but the meaning is different. When you say "could have", you are talking about something that is possible, but not necessarily true. When you say "must have", you are talking about something that is true. Arthur Rodway must have been going through a painful illness. But while he was going through a painful illness, he could have also been going through other things, related or unrelated.

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  • 'When you say "must have", you are talking about something that is true.' must just expresses a much higher degree of certainty about a proposition than could does -- I don't see how you can reason from that that the proposition cannot be false.
    – HeWhoMustBeNamed
    Jun 5, 2020 at 4:44

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