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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?

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Why would you do that? is a question in the present tense. And by convention the present tense is used for reasoning in general, so it might be applied to the past.

Why would have you done that? inquires specifically about the reasoning and conditions that may have affected your mood about doing something in the past (which may or may not have happened, or been done by you).

People are not always this deliberate. But not only does the second question ask after the past, the change in word order subtly reorients the interrogator in terms of skepticism. Consider the difference:

Me: I once boxed a jellyfish while skydiving.

Gullible Niece: You have done that?

Storyteller: I once arm wrestled a kangaroo.

My Slightly Older, Wiser Niece: Have you done that?

In the first case, I can probably keep telling her ridiculous nonsense. In the second case, the jig is probably up and it's time to find something more interesting to do.

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  • Why would you do that?

Implication: A person would do something if something else happened.

Formal grammar pairs would with simple past** or was/were:

  • I would see you tonight if you arrived tonight.

It is hypothetical, it hasn't happened. It is spoken in the present about something that has not occurred.

  • Why would you have done that?

That is for something that has already happened. Past conditional tense.

I would have done it if I could.

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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
    Commented Nov 16, 2019 at 2:28
  • @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
    Commented Nov 16, 2019 at 18:56
  • 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
    Commented Nov 16, 2019 at 21:06
  • @AIQ, it's the only option in this case, as we're not talking about a hypothetical.
    – Chris Mack
    Commented Nov 17, 2019 at 15:53
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    @sas08: “Why would have you done that” is not proper English, it reads merely as an incorrect syntax of your other phrasing.
    – Chris Mack
    Commented May 2, 2020 at 20:09

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