How to use "unless"

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.

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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.