If I were you = Refers to a hypothetical situation in the present or the future (subjunctive) and doesn’t refer to the past; it is a condition which is contrary to the fact, that is, I am NOT you:
If I were you, I would call the police.
Notice that the subjunctive ‘If I were you’ is currently going to be out of favour in modern English and has become obsolete for most people. So, by its very nature, it doesn’t have to be conjugated, and can go only in the following form: If I were you, I would call the police.
Now see this: If I were you, I would have called the police. (This construction is not in accordance with the Standard English). If I were … only indicates an imaginary or unreal situation.
Remember, ‘if I were you’ is also used when we give someone advice: If I were you, I’d probably call the police. (I think I’d call the police if I were you).
If I had been you = (also Had I been you) refers to a hypothetical situation in the present or the future (subjunctive); used when it’s irreversible: If I had been you (at some time in the past), I would have felt the same way. But, I wasn’t you, so, didn’t feel the same way:
If I had been you, I would have called the police. However, this construction (the subjunctive) is also going to become obsolete in modern English and can go on like:
If I were you, I would have called the police.
Also, see these:
If I had the money, I would have bought those shirts in the sale. (= as I am strapped for cash as ever, I couldn’t buy those shirts)
If I’d had the money, I would have bought those shirts in the sale. (= though I am well off, I didn’t have money with me at that time)
If I weren’t married, I would have proposed to Sarah. (= since I’m married, I didn’t propose)
If I hadn’t been married, I would have proposed to Sarah. (= since I was still married at that particular time (could be separated now), I didn’t propose)
If her parents were wiser, they wouldn’t have married her off to that guy. (= her parents are always unwise and are still the same)
If she had been wiser, she wouldn’t have married to that guy. (= she was in her teens then, couldn’t consider her welfare at that time)
