An article explaining the difference in meaning between if and unless in conditional sentences.
Unless = if … not. This is a good example of a 'rule of thumb' – one that is easy to remember and apply, but which is only a part of the whole story. Better to think of unless as meaning ‘except if…; except under these circumstances…’. So:
I’ll be at your place at 9.00, unless [= except if] the bus is late.
The front room was never used, unless [= except if] we had important visitors.
In the first sentence there is only one reason why I might be prevented from being at your place at 9.00, and that would be the bus being late. This is what Swan means when he said ‘A will happen if it’s not stopped by B.’ Likewise in the second sentence there was only one circumstance when the front room was used: when we had important visitors. But unless is unlikely here:
I’ll be angry if the bus isn’t on time.
In this sentence the bus not being on time is the reason for my being angry. That is what Swan means when he says ‘A will result from something not happening’. The following sentence I’ll be angry unless [= except if] the bus is on time means: the only thing that will prevent me from being angry is the bus being on time – which sounds strange to say the least. Here are some more examples like that, where unless isn’t possible:
If you don’t like it, you can leave.
If she wasn’t so bossy, she’d be quite nice.
If I didn’t know a word, I’d look it up.
Here is an exercise that attempts to discriminate between these two meanings of if…not. Which of these sentences could you re-write with unless?
1. If you don’t have an umbrella, I’ll lend you mine.
2. If you don’t take an umbrella, you’ll get wet.
3. He’ll fail his exam if he doesn’t study.
4. I’ll be very disappointed if he doesn’t study.
5. Will it be all right if I don’t wear a tie?
6. They won’t let you in if you don’t wear a tie.
7. If it doesn’t rain in August the tourists will be happy.
8. If it doesn’t rain this August, there will be water shortages.
9. In the old days people never travelled if they didn’t really have to.
10. You’d sleep better if you didn’t drink coffee before bed.
You can substitute if…not with unless in sentences 2, 3, 6, 8 and 9.
Notice how unless seems often to occur in the context of bad or unfortunate things happening: is this perhaps a clue to how it’s used – and a possible - more reliable – 'rule of thumb'?) A quick search of the COBUILD corpus wasn’t much help here, as the concordance lines are not long enough to give enough contextual information – but the following examples were thrown up:
...she would be killed unless she resigned from the post.
...threatened with violence unless she did as she was told.
...threatening to go to the police unless he is compensated.
...released by tonight unless police seek an extended detention.