Learner's definition of UNLESS
— used to say what will happen, be done, or be true if something else does not happen, is not done, or is not true
He will fail the course unless he gets a 90 on the exam.
Unless something is done, the species will become extinct.
I won't have an operation unless surgery is absolutely necessary. [=I will only have an operation if surgery is absolutely necessary]
“Will I have to pay?” “Not unless [=only if] you want to.”