Tuesday, August 15, 2017

modal verbs - 'Would have known' vs 'could have known'



“I’ve thought and thought about how the killer got in, and I’m sure he must have followed her inside when she came in that morning, because of Derrick Wilson leaving his desk and being in the bathroom. I thought Wilson ought to have been bloody sacked for it, actually. If you ask me, he was having a sneaky sleep in the back room. I don’t know how the killer would have known the key code, but I’m sure that’s when he must have got in.” (The Cuckoo’s Calling, by Robert Galbraith)




Would’ and ‘could’ are both used to indicate a possible thing, say OALD would #3 and could # 4. Then, can the ‘would’ in the novel be replaced with ‘could’ - ‘would' here doesn't seem to indicate irrealis but inference - with the meaning unchanged? Or would there be any semantic change if ‘could’ was used?



Answer



If someone gives you a present, you might exclaim either:



  • How would you have known it was my birthday?!

  • How could you have known it was my birthday?!


There is probable agreement among English speakers that "how+could" is the more fitting pair when there is something unlikely to happen--that did happen--and you are expressing surprise about it.


Consider:




  • I don't know how the killer would have known the key code, but he did!

  • I don't know how the killer could have known the key code, but he did!


My preference is for "could" as a more natural way of conveying surprise about the improbable event that happened. To my eyes it's more congruous with "...but he did!"


By corollary, this case:



  • I don't know how the killer would have known the key code, so he must not have!

  • I don't know how the killer could have known the key code, so he must not have!



I'm not sure if there's much to remark on the difference. What if the alliteration of "killer could" was reversed?



  • I don't know how the walrus would have known the key code, so he must not have!

  • I don't know how the walrus could have known the key code, so he must not have!


It seems less a clear winner, in any case. :-)


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