Dear Pivetta: I have forgotten all the grammar I once knew, and all our experts must be watching the Olympics. I would never use the "would" in your example. "I wish I could have known.." seems OK, or simpler "I wish I had known."
I'm a stranger here myself, and I hope you will be made to feel as welcome as I have.
"If I had known" is the way to go in the subordinate clause. "Would have" sounds right in the main clause. It has to do with what happened first and what came later.
Welcome to the Board Pivetta! For what it's worth, when I am truly rueful I say : "I wish I'd've known" which, I guess, is a contraction of "I wish I would have known" ... it's all too complicated, this language of "ours!" I tend to go with what sounds right but I wish I'd paid more attention in school.
Tsuwm will help you out ... now that he is no longer hiding his brilliant light under a barrel! (See "a nother promise" thread in Information and Announcements)
A hearty welcome to you, Pivetta! I'd sort of thought that 'I had known' was correct, and the other a sloppy but common idiom. Apparently I was wrong. Looked at your bio, Sweetie--you seem to have enough quirkiness to fit right in here! Re: your special interest, musick likes baseball pretty well, but you really ought to check in with AnnaStrophic and Faldage, for some real die-hard fan discussion. Just a suggestion.
While we are at it, you get my WELCOME and MaxQ gets my hearty thanks for expressing how I feel about this site:
I have stayed out of this thread to date because my grasp of grammar isn't worth ...(read MaxQ's post for the rest) ... ask more questions - that way I can learn vicariously.
Fans of mystery novels (I am one) refer to the "Had-I-But-Known School" of mystery writing, the greatest practitioner of which was Mary Roberts Rinehart. That's a rather old-fashioned way of expressing the same thing.