I goofed! Can I change my vote? ;-) (I saw something that wasn't there.) It should be: "If I'd known, I would have told you."
I voted "Acceptable sentence in some regions". Oh come on ! Surely it was acceptable in some regions...like The East End....init?
I know it's it acceptable in my region. Canada.
Well, if this is how you say "would have", then this is a perfectly acceptable SPOKEN English sentence. It's a language awareness more than anything else.
it is grammatically incorrect.
Terrible written sentence, but I understand where the misconception comes from ('would've'), so unless it's very clearly pronounced 'of' instead of 'uve', it's close enough when spoken.
If I'd 'have' known, I would 'have' told them. Let's look at where the apostrophe is... I'd = I would. If you say the initial part of the sentence without the apostrophe it should still make sense... "If I would known" which in this case it doesn't. "If I would 'have' known" makes sense, therefore it should be the same when the apostrophe is included. Also in the second part of the sentence, the use of the word 'of' is misplaced. In a past tense phrase, you must use past tense and in conjunction with the use of the word 'have' in the previous part of the sentence it also fits well with continuity which is very important.
Horrible english that doesn't make sense and drives me nuts.
@G: As a non-native speaker of English, G's commentary sounds a little erratic to me. I was taught that the past tense is to be used in the HEAD clause ("If I had known..") and 'would' in the SUBclause ("..., I would have told..."). Correct me, if I am wrong. So the sentence in spoken English would run: "If I'd known, I would've told you.", or even: "..., I'd have told you.", or perhaps: "..., I'd've told you." Not bad for a Dutchman, eh?
Oh, my G! I mixed up MAIN and SUB! Must be dotage...
So: past (perfect) tense in the SUBclause ("If I had known..") and 'would' in the MAIN clause ("..., I would have told..."). Sorry!