There are many kinds of conditional sentences. 'If I would have known' is not correct in any of them in standard BrE, though you will often hear 'If I'd've known' in informal conversation. Many British speakers conider the 'd to be a contraction of had .
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fivejedjonyou will often hear 'If I'd've known' in informal conversationCopy that. Once you start noticing this, it's just amazing how common it is. (A bit like "is is", but please don't get me started on that one.)
dave_anon If you allow the first of the following, it's difficult not to allow the fourthAnd therein lies the crux of the problem. I don't allow the first. And neither do countless others.
... If he would have appeared in court, ...
dave_anonwhat do you see as the difficulty?It's not standard to put 'would have' in an if-clause. The correct word is 'had'.