I took part in this thread because I had never heard or seen the use of that structure before.
I asked some teachers here and all of them reacted in the same way. I wonder what would happen if I decided to write mustnīt have been instead of canīt have been to show certainty about something in an international exam. I donīt think it would be accepted.
Iīve found this use in another grammar book:
mustnīt have = necessity in the past
e.g.: You mustnīt have had any driving convictions or you wonīt get the job.
- For Teachers