*un‧less is 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.
*not unless only if:
'Will you go with her?' 'Not unless she wants me to.'