I love how people from the UK say "had done" ... for example.
"He would not agree even if he had done."
I guess what I'm missing here is the "it"
"He would not agree if he had done it."
But more than just being a quirk it has a different kind of spiffy feel to it? "as if you would do" for "as if you'd ever do that"
I wonder if it's regional within the UK, or if it's just the US that's still using the extra words and just where English is going.
@futurebird Learning Swedish perhaps gives me an insight I might not otherwise realise, as a native Old World English speaker (Ireland, UK, South Africa).
In Swedish we have the supine tense, which modifies the second verb in this construction your showing here:
Jag har gjört = I have done. Gjört is göra in the supine, and göra means to be. It seems correct also to say Jag här gjört det, too, though. The point being that in Swedish it's formally defined as a tense in this way, with its own verb modification system. So I guess the UK English construction feels a little more intelligible to me from seeing how it works in a closely related language!