[bù rán]
(不是这样) not so; not the case; no
Actually this is not so.
其实不然。
I've got to leave a bit early, otherwise I'll miss the train.
我得早点去,不然就赶不上火车了。
Either be there on time, or don't go at all.
要去就别迟到,不然, 就甭去了。
No, it's not as simple as that.
不然, 事情没有那样简单。