double negation: unless followed by a negative verb form
"We will not cancel the game if the weather gets better."
Replacing "if" with "unless", you end up with either:
"We will cancel the game unless the weather gets better."
or
"We will not cancel the game unless the weather does not get better."
Is the latter sentence an absolute no-no? Or even a no-no-no!
Solution 1:
I think what makes the example sentence unpleasant is that, in addition to the three words in bold type, "cancel" also has a negative meaning. My preferred formulation of the intended meaning would be "The game will be played unless the weather stays bad."