if not 是什么意思?

相当于 如果。。。就不。。。
例如
--I think if John insists on going to the States, we have to find someone else to replace him.
--Not if I can talk him out of it.

如果约翰坚持要去美国的话，我们就得找人替代他了
如果我能说服他不去，我们就不必找人了

如果不

如果不，否则