You will lose marks in the exam unless you improve your handwriting.大意为：除非你提高你的书写,不然你就会在考试中丢分
或
You won't lose marks in the exam unless you don't improve your handwriting.大意为：你不会再考试中丢分,除非你不提高你的手写.
都一样,只不过一个是双重否定.