If Fred didn’t leave here before five, he mustn’t be home yet.

[touchstone]

There’s one mistake in the following sentence. Find and correct it.
If Fred (A)didn’t leave here (B)before five, he (C)mustn’t be home (D)yet.
(from a test paper)

The answer key is C: mustn’t →can’t.
But I think there is something wrong with the question sentence and it should be: if Fred hadn’t left here before five, he couldn’t have been home yet. What do you say?

Thanks.

 

[touchstone]

Thanks, Piscean. So the answer key is correct?

 

[Matthew Wai]

If Fred didn't leave (1. possible) - it is possible that he didn't leave.
............................(2. conceded) - and we know that he did leave.

How could it be possible that he didn't leave when we know that he did leave?
We know that he did leave, so 'didn't leave' is counterfactual rather than possible.

 

[Matthew Wai]

If Fred didn't leave (1. possible) - it is possible that he didn't leave.
............................(2. conceded) - and we know that he didn't leave.

I think we don't know if he didn't leave.
If we know that he didn't leave, it is factual rather than possible that he didn't leave.