Step 1: Rule of “unless.”
The word “unless” means “if not.” Therefore, it already expresses a negative condition. Using “will not” with it makes the sentence double negative.
Step 2: Correction.
Incorrect: “Unless you will not work hard, you will not succeed.”
Correct: “Unless you work hard, you will not succeed.”
Step 3: Conclusion.
The incorrect part is “will not.”
Final Answer:
\[
\boxed{\text{will not}}
\]