We use the conjunctionunlessto mean ‘except if’. The clause which followsunlessis a subordinate clause (sc): it needs a main clause (mc)to make a complete sentence.
Whenunlesscomes before the main clause, we use a comma:
Unless[SC]it rains,[MC]we’ll go for a picnic by the river tomorrow.(We’ll go for a picnic by the river tomorrow if it doesn’t rain.)
When the main clause comes first, we don’t need a comma:
[MC]They won’t comeunless[SC]you invite them.
Unlessis a conditional word (likeif), so we don’t usewillorwouldin the subordinate clause:
UnlessI hear from you, I’ll see you at two o’clock.
Not:Unless I’ll hear from you…
See also:
If
Conditionals
Unlessandif … not
Unlessandif … notboth mean ‘except if’:
We could eat at Siam Smileunlessthey’re closed on a Monday.(orWe could eat at Siam Smileifthey’renotclosed on a Monday.)
I’ll make dinnerunlesssomebody else wants to.(orI’ll make dinnerifnobodyelse wants to.)
Can you turn the radio offunlessyou’re listening to it?(orCan you turn the radio offifyou’renotlistening to it?)
We don’t useunlessfor things that we know to be true:
You won’t be able to get a ticket for the matchunlessyou’re prepared to pay a lot of money for it.(The speaker doesn’t know if you’re prepared to pay a lot of money for a ticket.)
I don’t know what we would have doneifwe hadn’tseen you.(We did see you.)
Not:I don’t know what we would have done unless we’d seen you.
In speaking, we useunlessto introduce an extra thought or piece of information:
He didn’t even know about the crash –unlesshe’d heard about it on the radio.
A:Oh look. Neil next door’s got a new car.
B:Unlessthey’ve got a visitor.
Typical errors
We don’t useunlesswhen we meanif:
Pete will driveifAlex can’t.
Not:Pete will drive unless Alex can’t.
We don’t usewillorwouldin the clause afterunless:
Unlessyou pay now, we can’t guarantee you a ticket.
Not:Unless you’ll pay now…
See also:
Conditionals: other expressions (unless, should, as long as)
If
Unless