Unless + present

Example :

               You'll be sick unless you stop eating. (= You will be sick if you don't stop eating)
                I won't pay unless you provide the goods immediately. (= If you don't provide them I won't pay)
               You'll never understand English unless you study this grammar carefully. (= You'll never understand if you don't study...)


Unless + past perfect

Example :

               Unless he was very ill, he would be at work.
                I wouldn't eat that food unless I was really hungry.
               She would be here by now unless she was stuck in the traffic.


Unless + past

Example :

               Our marketing director would not have signed the contract unless she'd had the company legal expert present.
                I wouldn't have phoned him unless you'd suggested it.
               They would have shot her unless she'd given them the money.