If you read many books written by British authors or watch many British films, you will be familiar with sentences like these from Susan Howatch's novel Absolute Truths:
"You could have done, but you didn't."
"I would have done, but I never got that far."
Those lines always jar me a little when I come across them. That's because Americans omit the done in similar sentences. We would say, "you could have, but you didn't" or "I would have, but I never got that far."
So who's right? Do Americans leave out a word that should be there, or do the British insert a word that isn't necessary? I think you could make an argument both ways.
We Americans understand one another perfectly, so obviously the word done is not really necessary in those sentences. It is always implied, however. The British put the word in their sentences rather than just let listeners and readers assume it is there.
Here on the west side of the Atlantic we do use other verbs in similar sentences. We say, "it would have been" or "I could have gone." Sometimes we leave them out, but just as often we include them. Yet only the word done seems strange to us when we hear it or see it in print in these kinds of sentences. Or am I alone in this?