The structures 除非…，否则… and 除非…，不然…
The structures 除非…，否则… 
and 除非…，不然… are used in the same way. They mean “unless...”.
These structures indicate that the result "B" can’t occur under condition A. Under any other conditions, “B” would occur. The result usually has a negative connotation.
and 除非…，不然… are used in the same way. They mean “unless...”.
These structures indicate that the result "B" can’t occur under condition A. Under any other conditions, “B” would occur. The result usually has a negative connotation.Scheme
除非
condition (A)
否则/不然
result (B)
Pay attention
A sentence with this structure usually has two parts. The subjects are placed after 除非 and after 否则/不然.
Examples
除非您有会员卡 ，否则不能进去
You can't enter unless you have a membership card
chúfēi nín yǒu huìyuánkǎ, fǒuzé bù néng jìnqù
除非提前从家里出发，否则我们会迟到
If we don't leave the house early, we'll be late
chúfēi tíqián cóng jiālǐ chūfā, fǒuzé wǒmen huì chídào
除非他有证据 ，不然谁会相信？
If he has no proof, then who will believe him?
chúfēi tā yǒu zhèngjù, bù rán shéi huì xiāngxìn