Unless means the same as if...not. Like if, it is followed by a present tense, a past tense or a past perfect (never by 'would'). It is used instead of if + not in conditional sentences of all types:
Type 1: (Unless + present)
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)
never understand English unless you study
this grammar carefully. (= You'll never understand if you don't study...)
Type 2: (Unless + past)
very ill, he would be at work.
- I wouldn't eat that food unless I was
- She would be here by now unless she was
stuck in the traffic.
Type 3: (Unless + past perfect)
- Our marketing director would not have signed the contract unless she'd
had the company legal expert present.
wouldn't have phoned him unless you'd
would have shot her unless she'd
given them the money.