I have found a restriction on the use of "unless" which was difficult to explain to my students (advanced ESL).
The student's sentence was:
I would have had to go to the bank unless you had lent me the money.
Several others produced similar sentences. How can we explain the difference in acceptability between:
They'll kill her unless she gives them the money.
*They would have killed her unless she gave them the money.
- They wouldn't have robbed her unless they were really poor.