counterfactual conditionals

[AAllan at AOL.COM]

<< it's probably too much to hope that the WOULD-counterfactual has
caught the attention of some prescriptive usageists.  >>

Merriam-Webster's Dictionary of English Usage, the OED of usage, is always
worth a look. s.v. "would have":

>>>The use of "would have" in place of "had" in the protasis of a conditional
sentence (as in"If they would have come earlier, we could have left on time")
has been cited as an error in books on usage since at least 1924. . . .

[goes on to give citation & discussion]

Our evidence indicates that it does not occur in standard writing that finds
its way into print, but it is notorious in student writing and therefore a
staple of college handbooks even today.<<<<

- Allan Metcalf