un ‧ less S1 W1 /ʌnˈles, ən-/ BrE AmE conjunction
[ Date: 1400-1500 ; Origin: on less than 'on a lower condition than' (1400-1500) ]
1 . used to say that something will happen or be true if something else does not happen or is not true:
Unless some extra money is found, the theatre will close.
I think you should complain – unless, of course, you are happy with the way things are.
He won’t go to sleep unless you tell him a story.
I can’t leave her unless I know she’s all right.
2 . not unless only if:
‘Will you go with her?’ ‘Not unless she wants me to.’
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GRAMMAR
unless, if ... not, in case, or (else)
Use unless to say that something will happen or be true if something else does not happen or is not true. Do not use 'will' or 'shall' after unless . Use the present tense or present perfect:
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Businesses will not survive unless they satisfy (NOT will satisfy) their customers.
If you want to mention something that did in fact happen or is in fact true, use if ... not :
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If he had not tripped, he would have won (=but he did trip) .
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I would go out if it wasn’t raining (=but it is raining) .
Use in case when talking about something that is or should be done because something might happen:
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Take a sweater in case you get cold (NOT unless you get cold).
Use or or or else to say what bad thing will definitely happen if something else does not happen:
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You’d better go, or else you’ll miss the train (NOT unless you miss the train).
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THESAURUS
▪ if used when talking about the possibility that something might happen or be true:
He faces a long prison sentence if the court finds him guilty.
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If scientists’ predictions are correct, average global temperatures could rise by 6 degrees.
▪ unless if something does not happen, or if someone does not do something:
The star is difficult to see unless the sky is very clear.
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Doctors said they could not treat the boy unless his parents gave their permission.
▪ whether or not used when saying that it does not matter if something happens or not, or if something is true or not:
Most people will get better on their own, whether or not they receive medical treatment.
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I’m still going, whether she likes it or not.
▪ otherwise used when saying that there will be a bad result if someone does not do something, or if something does not happen:
Drink plenty of water – otherwise you will become dehydrated.
▪ in case in order to deal with something that might happen:
She did not think it would rain, but she took her umbrella just in case.
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It is best to keep a medical kit ready in case of emergency.
▪ as long as/provided that only if something else happens or is true:
Visitors are welcome, as long as they bring their own tent.
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Anyone can join the course, provided that there is space available.
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As long as you can find a computer, you can access an internet-based bank account wherever you are.
▪ on condition that used when you agree to do something only if someone first agrees to do something else:
He was offered the job on condition that he went on a month-long training course.