I wish I could explain my following answer in terms of grammar rules. It is common for the past perfect verb to be followed by the simple past tense. But as soon as the sentence involves conditional terms or verb forms, such as 'if and would', I am unsure what the rules dictate. Therefore, my answer is simply based on what I've heard over the years (past perfect followed by the present perfect): If I HAD KNOWN what would happen, I would have never come here. (OR I would never have come here.) If I HAD KNOWN what would happen, I would have never done it. (OR I would never have done it) Can anyone tell us the rule that applies?

Slight ajustment: "If I knew what would have happened, I would have never came here." "If I l knew what would have happened, I would have never done it."