After "could"

In U.S. English it would normally be "...could have". I believe British might be "...could have done".

Thanks a lot, Philip!!

If it is normally "could have", then the sentence must be ungrammatical?

I feel that we can't use "could" instead of "could have"....

Yes, the original sentence is ungrammatical.

After some searching, I found out that "would have PP if * could." is more common than "could have.".

Maybe it's because "would have" already came before, and to avoid repetition, even though that's ungrammatical?

Some kind of, not caring much about grammar, but caring about rhythm, sound or beauty?

Maybe this is one example that grammar can't explain all the varieties i

pructusWhat is omitted after the “could”?“have stood up” or “stand up”?

'have stood up'

If I were saying it, I would probably add the "have": He would have stood up if he could have.

Typical in casual conversation:

— Why didn't he [stand up / sit down / get out of the way / run / stop / ...]?
— He would have if

Oh, I see... I see...
Thanks so much, CJ!!

pructusBecause "would have PP" is already signifying the event is in the past, so only "could" is enought to convey the situation is in the past?

I see it as an incomplete part of the second half of a 3rd level conditional. That's the only reason I find it ungrammatical. Of course, it is perfectly understood.

I see... I see...
Thanks again, Philip!!