Unless a le même sens que if...not. Comme if, unless est suivi d'un temps présent, d'un temps passé ou d'un temps passé parfait (jamais par le conditionnel). Unless est utilisé à la place de if...not dans les phrases conditionnelles de tous les types. L'ordre des propositions n'a pas d'importance dans les phrases qui utilisent unless.

Type 1 conditionnel: Unless + présent
Avec If L’équivalent avec Unless
You will be sick if you don't stop eating. You'll be sick unless you stop eating.
I won't pay if you don't provide the goods immediately. I won't pay unless you provide the goods immediately.
If you don't study dilligently, you'll never understand trigonometry. Unless you study dilligently, you'll never understand trigonometry.
Type 2 conditionnel: Unless + passé
Avec If L’équivalent avec Unless
If he wasn't very ill, he would be at work. Unless he was very ill, he would be at work.
I wouldn't eat that food if I wasn't really hungry. I wouldn't eat that food unless I was really hungry.
She would be here by now if she wasn't stuck in traffic. She would be here by now unless she was stuck in traffic.
Type 3 conditionnel: Unless + passé parfait
Avec If L’équivalent avec Unless
Our director would not have signed the contract if she hadn't had a lawyer present. Our director would not have signed the contract unless she had had a lawyer present.
I wouldn't have phoned him if you hadn't suggested it. I wouldn't have phoned him unless you'd suggested it.
They would have shot her if she hadn't given them the money. They would have shot her unless she'd given them the money.