There has been many decades... or There have been many decades... Are they interchangeable and is the first one wrong?
There have been many decades. No, they are not interchangeable here. The plural verb "have" is required because its object "many decades" is plural. PaulM
Correction to my last answer. I should have said 'because the complement of "been" is plural'. PaulM
Your help is invaluable, really.