What's the difference between "If I had done" and "If I did"?

Question

1. If I had known she was coming, Id have come too.

1st Question: I really understand this sentence but what if I say it like “If I knew she (would come or not or was coming or not?), I'd come too.”

Am I wrong to think that the second one might be incorrect because you haven't yet known whether she will come or not?

2. (a) If I do it, it will be a mistake.
   (b) If I did it, it would be a mistake.

See! Both of them are related to the future somehow.
2nd Question: What's the difference? Is it the difference of the gap of time? I think in sentence (a), it indicates that he's gonna do something right away. So, how about the sentence (b)?

Answer 1

Regarding the first question, both are used to talk about hypothetical situations, however, the grammatical structure underlying in If I had known carries a nuance of regret (maybe the speaker was keen on her and would've liked to hang out with her). Although, if I recall correctly, using the following wording is more common Had I known....
In contrast, If I knew alludes to the present, referring to an action that is still untrue in the present.
In fact, both these grammatical structures have a name: the past and present unreal conditionals (This is what I do recall, if anyone could confirm or correct this I would gladly correct my answer).
All in all, the difference is in that slight nuance.

Regarding the second question, I do believe that the difference lies on the likelihood of the discussed hypothetical situation, from the viewpoint of the speaker.
Let's imagine that someone is asking you to do something that may end up having negative consequences. Let's also imagine you have some feelings for the person that is requesting you to do such a thing. For this reason, you might be considering doing it for the love you feel, in this case, you may say

If I do it, it will be a mistake.

despite the fact that from a logical view, it should not be done under no circumstances.
However, if you clearly thought it was a stupid idea, then you may say

If I did it, it would be a mistake.

Answer 2

1st question: I don't understand what usage you're asking about -- it seems the words in the parentheses include some optional text, and I don't know what options go together.

2nd question: I don't get any 'right away' meaning from the first form. They state the same thing, one of them from the standpoint of present tense then future tense (speaking in present tense about the deed, then future about the outcome); in the second, both in past tense (speaking from a time beyond when the deed might have been done). There isn't much, if any, difference in meaning here.

Answer 3

I was also curious about the answer, so I searched for stuff about tenses in an if-clause. Apparently, some people were also curious about this topic too.

past simple vs past perfect in conditionals (← Question 2)

According to the answer to this question, both "If I had known she was coming" and "If I knew she was coming" means a very similar concept, but (I think) the former one is something we can use for the events that have happened far past (like retrospecting the event that she might have come), but the latter one is for the events that are happening right now (or just a moment ago). (← Question 1)

I'm no native speaker so it's very likely that this is somehow incorrect, but that's what I'm understanding here.