Look at these examples and decide what the rules are:
e.g. If you don't smoke, you won't die of cancer.
Unless you smoke, you won't die of cancer.
2. If you don't give up smoking, you will die.
Unless you give up smoking you will die.
3. If you don't water plants, you won't get leaf disease.
Unless you water plants, they will die.
4. Unless you eat more, you will die.
If you don't eat more, you will die.
An explanation which you might like to challenge, could be:
a positive 'unless' clause cannot be followed by a negative main clause, without reversing the meaning.
A negative 'unless' clause cannot be followed by a positive Main Clause without reversing the meaning. However, this is not always true! Look at 3 above!
Grammarcat's advice: don't be so complicated! Keep it simple! You can use ' provided that', 'but for', 'on condition (that)', 'assuming (that)' - there are lots of other ways to get out of trouble!
See The A - Z of English Grammar & Usage for the rules