除非 … 否则 — unless

规则 除非 A，否则 B = unless A, then B. B is what follows if A does NOT happen.
除非你去，否则我不去。
除非 A，才 B = only if A. Same meaning, but the second clause says what happens when A DOES hold.
除非你去，我才去。
除非 states the ONE condition, close to 只有; 如果 merely floats a possibility.
否则 = 不然, opening the second clause with the bad consequence: 快走吧，否则就迟到了。