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)