Why would he do/have done that?

[Vladv1]

If a person did something stupid in the past, should I say "Why would he have done that?" or "Why would he do that"? Can "why would he be doing it" be used to refer to something stupid that is progressing in the present? The examples are mine.

 

[Tarheel]

When you say somebody did something you are speaking in past tense. You don't need to add "in the past".

If you are puzzled about something somebody did, you could say, "Why did you do that?" Or: "Why did he do that?"

 

[Annabel Lee]

If a person did something stupid in the past, should I say "Why would he have done that?" or "Why would he do that"?

In my experience as a native speaker of American English, each of those sentences works in that type of case:

A: Bob flipped off a police officer yesterday.​

B: Really? Why would he do that? / Really? Why would he have done that?​

The "would he have done" version is stronger, implying that one can scarcely believe that Bob did do such a thing.

Can "why would he be doing it" be used to refer to something stupid that is progressing in the present?

Yes, it can:

[context: looking out the window]
Why would Bob be standing naked in his driveway in broad daylight?​

 

[Vladv1]

In my experience as a native speaker of American English, each of those sentences works in that type of case:

A: Bob flipped off a police officer yesterday.​

B: Really? Why would he do that? / Really? Why would he have done that?​

The "would he have done" version is stronger, implying that one can scarcely believe that Bob did do such a thing.

Yes, it can:

[context: looking out the window]​

Why would Bob be standing naked in his driveway in broad daylight?​

Can I say "Why would he do that?" it the action happened 100 hundred years ago?

 

[emsr2d2]

Can I say "Why would he do that?" if the action happened 100 hundred years ago?

Yes.

 

[Tarheel]

@Vladv1 Where did you see the phrase "progressing in the present"?