You can use "if not" and "unless" in place of each other when we are saying 'except if'. Unless is 'stronger' than if not, and we sometimes use it in 'threats'. For example; If you don't change your mind I won't be able to help you. (= Except if you change your mind..) Unless you change your mind, I won't be able to help you. (= Except if you change your mind..) 2. We cannot use unless in place of if not when if not doesn't mean 'except if'. for example; I'll be surprised if he doesn't win. She'd be better company, if she didn't complain so much. 3. We often use unless (never if not) to introduce an afterthought. For example; I couldn't have got to the meeting - unless, of course, I had caught an earlier train.