Unless = if not, except when.   ‘ Unless ‘ is not used in imaginary/unreal or hypothetical situations. ‘ Unless ‘ negates a sentence  and therefore no further negation is required by adding No/None/Not etc as in the case of ‘ If’.

• Unless you work hard, you will not pass the exam.
• I will not go to the party unless I am invited.
• If he weren’t so stupid , he would understand it. ( here we don’t use ‘ Unless’ as it is an imaginary or hypothetical situation)
• Unless it rains, he goes for a walk everyday.
• I will lend you my camera unless my brother wants it for his use.
• If she hadn’t misbehaved, I would have helped her. ( you cannot use ‘Unless’ here)
• Students are not allowed to conduct the risky experiment themselves unless they are under the supervision of a teacher.