‘UNLESS’ expresses a condition, It is always used in the negative sense. Thus ‘NOT’ is never
used with ‘unless’.
Ex.-Unless you do not labor hard, you will not pass. (Incorrect)
Unless you labor hard you will not pass. (Correct)
‘Until’ expresses time. It has a negative sense and thus ‘not’ should never be used with it.
Ex.-Wait here until I do not return. (Incorrect)
Wait here until I Return. (Correct)