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.