Unless you will not work hard, / you cannot / clear the written / examination.

'Unless' already carries a negative sense (if not), so adding 'not' creates a double negative. The correct form is 'Unless you work hard'. The trap is the future 'will', which is also unnecessary in a conditional clause. Match the tense in the if-clause with the main clause: past perfect with would have, simple past with would, and simple present with will.