除非...不然/否则 means "unless X, otherwise Y." It states a necessary condition and what happens if it is not met.
Necessary condition - X must happen to avoid Y
Warning or consequence - What happens if the condition fails
Unless you apologize, otherwise I will not forgive you.
否则 is slightly more formal than 不然, but they are interchangeable.
除非 introduces the ONLY exception
- The main clause usually contains a negative
不然 and 否则 are interchangeable
- 不然 is more colloquial
- 否则 is more formal/written
Can omit the second part if the consequence is clear from context
Unless you apologize, otherwise I will not forgive you.
Unless it rains, otherwise we will go hiking tomorrow.
Unless there is an emergency, he will not take leave.
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