If + would = wrong

Hello,

This is a nice web site, I just discovered it and I might need it in the upcoming weeks.

I'm french, and I have a german friend with whom I speak both English and German. German is his mother tongue, so I cannot really say anything about the mistakes he makes, however we fight a lot about English grammatical rules.

I keep telling him, and I'm sure about it, that "If I would have known" is completely wrong, and that he should be saying "If I had known".

Other exemple:

" If i would speak Japanese, I'd move to Tokyo" for "If I spoke Japanese, I'd move to Tokyo"
That's a typical German mistake ( Wenn ich Japanisch späche,...)

He keeps telling me that in US/ Canada, everybody says so and it's acceptable. It does hurt my ears whenever I hear that and I try to correct by telling him "If + would = wrong!!!"

Then quite ironically, some days ago I told him "If you wouldn't accept my help when I offered it, why would I help you now?" or something like that... And of course, he didn't miss the "If you would"...

I would appreciate some help... What is correct, and what's not?

I know the rule:

If I eat, I will sleep.
If I ate, I would sleep.
If I had eaten, I would have slept.

How can I justify, explain him? What about other significations of would (past=> "I said I wanted to sleep but they wouldn't stop shouting") etc...

Thank you very much for your help, I hope I explained my problem clearly enough.

W.