Will Have Known or Knew?

In a word,if"knew"is talking about the past,"have known"is talking about the past of the past in that way,right? In addition,just use"knew" when I speak. I think I get that. In a word,if"knew"is talking about the past,"have known"is talking about the past of the past in that way,right?

How do you use know knew and known?

'I have known her since I was young' is the only correct sentence out of the three, I know her - present tense - I know her now. I knew her - past tense - I knew her then.

Is would have known correct?

that there is no circumstance in which it is correct to say 'if I would have known'. 2. that some people say it quite frequently.