B has done something bad that A hadn't ever expected B to do so A is asking B
1- A: Why would you do that? I don't understand.
2- A: Why would have you done that? I don't understand.
What is the difference between these two options?
Firstly the second sentence needs rearranging:
Why would you have done that? I don't understand.
The sentence:
Why would you do that? I don't understand.
This sentence asks why B would do something, and sort implies that B would do it again, as though it were a part of who he is. It's much more direct, i.e., directed at B as a person.
Why would you have done that? I don't understand.
This sentence places the emphasis in the past, at the time when B did the thing. It expresses curiosity and a desire to understand with regards to what B was experiencing at that time, and urges B to investigate not so much himself, but what he was experiencing at the time, or what might have influenced him to do such a thing at that time.
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"... places the emphasis in the past, at the time when B did the thing" is that the only interpretation? We use "would have" with hypothetical scenarios too, then why is that not an option? – AIQ Nov 16 '19 at 2:28
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@Chris Mack Thank you but as far as I know native speakers also use "why would you do something" for situations that happened in the past. So what is the difference between them when "would you do" is used as I explained above? – Talha Özden Nov 16 '19 at 18:56
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The difference in emphasis comes from the words "do" and "done", with "do" implying that this is something characteristic of B, and less of a point in time. It is referring to the past event, but directed more at B's character than at the past event. – Chris Mack Nov 16 '19 at 21:06
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@AIQ, it's the only option in this case, as we're not talking about a hypothetical. – Chris Mack Nov 17 '19 at 15:53
