The accident needn't have happened if only Tom had got his lights repaired.

[Nonverbis]

Advanced Grammar in Use by Martin Hewings



What troubles me is item 3.

The key:



Well. This look strange.

My variant was: "needn't have been settled". I mean that they would not have negotiated on the recovery of damages. I'm not sure about whether my variant is clumsy or not.

But their variant just drives me crazy. "Needn't" is about necessity. It is not about a probability.The accident may/might/could not have happened - would be Ok. But they are not from this unit.

Could you clarify this matter for me: whether the key is correct or not?

 

[jutfrank]

When I did number 3, the answer that came instantly to mind was wouldn't have happened.

I'd argue that the answer given is not correct. It kind of sounds like something someone might say, but it doesn't actually make sense with a following condition clause.