Unless means the same as if...not. Like if, unless is followed by a present tense, a past tense, or a past perfect tense (never by a conditional). Unless is used instead of if...not in conditional sentences of all types. The order of the clauses doesn't matter with sentences using unless.
Type 1 conditional: unless + present tense
| With if | Equivalent with 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 conditional: unless + past tense
| With if | Equivalent with 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 conditional: unless + past perfect
| With if | Equivalent with 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. |