This question ['Would have known' vs 'could have known'][1] is one of the most interesting questions I have encountered. I did a research about its use. According to [*Ilse Depraetere 2009*][2], the function of *have* in sentences with *could* + perfect infinitive either communicates **anteriority** (as in the example in (1)) or contributes to the establishment of **counterfactual meaning** (as it does in (2)), which are the main functions standardly attributed to *have* in *could* + perfect infinitive constructions. And the paper highlights a special use of *could* + perfect infinitive in which *have* performs neither of the two functions (as it does in (3)). > 1) Michael scrutinised the window display in the hope of discovering > what had so held the child’s attention. Surely **it couldn’t have been > the dresses**. > > It could be paraphrased as “surely it **was**n’t possible for it to **have > been** the dresses”. > > 2) “How long did you spend recording the album?” – “Probably about a > month of recording and a few weeks of tweaking. **If I’d had a deadline > it could have been quicker.** > > It could be paraphrased as "If I had been given a deadline, it **would > have been** possible for me to **have finished** the album more quickly". > 3) **How is it that the Secretary of Defence could have remained at his desk until the crash?** Whose responsibility is it to relay emergency situations to him? > It could be paraphrased as "How **was** it possible for the Secretary of Defence to **remain** at his desk?" I suspect that in the linked question "I don’t know how the killer could have known the key code, but I’m sure that’s when he must have got in", *have* belongs to the second category of its functions. We could roughly paraphrase it as "I don’t know how the killer would have been able to know the key code, unless somebody else had told him". Do I understand it correctly? And I get some examples below from the Internet, with my own explanations added. Which of them could be substituted with "would have known" or "knew" without considerable change in meaning? Please help me walk through them. >> **#1.** "My name is Elijah," the man says, inspecting her with dark eyes. > "Your son – Malachi, is it? – asked me to help you." > **...** >He continues, "Like you, I'm a vampire, although **I don't know how your son could have known** (I'm a vampire) when he asked me to help you." >https://www.fanfiction.net/s/10170561/1/Hope-s-What-We-Crave Here the speaker is expressing surprise. We could roughly paraphrase it as "I don't know how it is possible for your son to have known..". I think *have* belongs to the first category. Using 'would' wouldn't fit with the context. And why not just simply say "I don't know how your son knew"? It seems neater. >> **#2.** *You're going to be playing the 2012 Vans Warped Tour. How excited are you? What are you looking forward to the most? Are there any other bands you're hoping to meet on the tour?* >>We are absolutely over the moon! I remember the day of our first show I said all I wanted was to play Warped Tour some day. **I don't know how much I could have known about the festival at the time** but I knew all my favourite bands played it so it feels really special to have reached that goal. We're all looking forward to catching up with the bands we've toured with and met over the past year of touring! >http://www.culturebrats.com/2012/02/seven-questions-in-heaven-with-tonight.html I'm not quite sure about the function of *have* here. Is it possible to use 'would' instead? I would reword it as "I don't know how much I knew about the festival at the time". >> **#3.** I remember the day the transcript came > and, that day, I was kinda freaking out that it was gonna come in the > mail. I was checking the mailbox, and I got the mail before my parents > could see it. They were blindsided by this. They had no idea that this > was going on. I did everything I could to lie to them and tell them I > was doing well. **I don’t know how they could have known.** They > weren’t there. I was away at school, so it wasn’t like they saw me > every day. There’s really no way they could have known. But in any > case, I took the transcript from the mailbox and I hid it. >http://livethroughthis.org/matt-fried/ I think *have* here expresses counterfactuality; the parents didn't know his bad results. Using 'would' would work, I suppose. >> **#4.** That spring I met a girl, or more precisely she met me. I was what is usually described as shy. In truth I was already somewhat socially and emotionally retarded. I had never really even talked to a girl. While my non-user and “normie” peer group were out going to social functions and starting to develop into young adults, I was always loaded and making no attempt at emotional or social growth. I avoided facing my fears and insecurities by not facing the real world. **I don't know how she could have known the only way I would have been able to talk to her**, but she found it by offering to smoke a joint. This was heaven for me. I didn't have to do anything—the pot did it for me. >https://www.marijuana-anonymous.org/book/our-stories/my-best-thinking-got-me-here It seems to be a hypothetical scenario here. I suspect 'would' is a possible alternative for 'could' while 'knew' couldn't be used. >> **#5.** I have some interest in collecting cards from TV shows and movies, and the current Panini set has been floating my boat a little, too. But **I don't know how Dave could have known that**. This is a pretty awesome bit of history. Dave sent many more cards. I don't have the time or the patience to show all of them. Rest assured, you'll see some of them in future posts. >http://nightowlcards.blogspot.com/2015/09/irregulars.html I think *have* expresses anteriority, as in **#1**. We could roughly paraphrase it as "I don't know how it is possible for Dave to have known..". Seems 'would' couldn't work here. Again, I would just say "I don't know how Dave knew that". Any different connotation conveyed? [1]: http://ell.stackexchange.com/questions/32752 [2]: http://perso.univ-lille3.fr/~idepraetere/Lille_III/Publications_files/Depraetere_2009.pdf