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
