Употребление "Unless"

Unless значит то же, что и if...not. Как и в случае с if, после unless идёт настоящее время, прошедшее время или прошедшее совершенное время (но никогда не условное). Unless используется вместо if...not во всех типах условных предложений. В условных предложениях с unless порядок простых предложений не имеет значения.

1-й тип условия: Unless + настоящее время
Употребление с If Эквивалент с 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.
2-й тип условия: Unless + прошедшее время
Употребление с If Эквивалент с 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.
3-й тип условияl: Unless + прошедшее совершенное время
Употребление с If Эквивалент с 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.