Use "Unless" instead of "If" and versa:
4. Unless he had drunk alcohol, he would have passed the contest.
-> If __________________________________________________
5. You'll lose your job if you don't change your attitude and become friendlier.
-> Unless ______________________________________________
6. These flowers will die if nobody waters them.
-> Unless _______________________________________________
he hadn't drunk alcohol, he would have passed the contest.
you change your attitude and become friendlier, you'll lose your job.
somebody waters these flowers, they will die.