Uso de "unless"

"Unless" significa lo mismo que if + not. Al igual que "if", "unless" va seguido de un verbo en presente, pasado o "past perfect" (nunca de un condicional). "Unless" se utiliza en lugar de if + not en oraciones condicionales de todos los tipos. Cuando empleamos "unless", el orden de las proposiciones no importa.

Condicional tipo 1: "unless" + presente
Con "if" Equivalente con "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.
Condicional tipo 2: "unless" + pasado
Con "if" Equivalente con "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.
Condicional tipo 3: "unless" + "past perfect"
Con "if" Equivalente con "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.