We can say:
"I can't bake a cake unless I get some flour. "
We should say:
"I can bake a cake provided that I get some flour"
I hope someone can explain what the rule is for not being able to say:
"I can't bake a cake provided I can't get flour."
I think it has something to do with a requirement or proviso, but I'm in difficulty figuring out the precise rule. Thanks!