When 'if.not' and 'unless' are interchangeable

Both if not and unless can be used in negative Type 1 conditionals

without a noticeable change of meaning If you don't change your mind I won t be able to help you Unless you change your mind I won t be able to help you

However unless is stronger than If not and is sometimes preferable

eg in an ultimatum Unless the management improve their offer there II be a strike

14.20.2 When we cannot use 'unless' in place of if...not'

Unless cannot replace if not in a Type 1 sentence like

/ // be surprised if he doesn't win This is because unless always means except on the condition that so we cannot normally use it to refer to unreal situations

She d be better company if she didn't complain so much

14.20.3 When we cannot use if...not' in place of 'unless'

We often use unless in past references to introduce an afterthought The unless clause follows the main clause and is usually separated by a dash rather than a comma

/ couldn t have got to the meeting on time — unless of course I had

caught an earlier train This means the speaker didn t get to the meeting He could only have done so by catching an earlier train If we use if not in place of unless in the above sentence we get

/ couldn t have got there if I hadn't caught an earlier train The sentence now conveys the exact opposite meaning the speaker did get to the meeting because he did catch an earlier train

14 Conditional sentences

If and 'unless' clauses in short answers

Note how if-clauses and un/ess-clauses can occur in short answers: Will you help us with all this re-decorating? - Yes, if I can No, not unless you pay me