UNLESS

Unless means the same as if...not. Like if, it is followed by a present tense, a past tense or a past perfect (never by 'would'). It is used instead of if + not in conditional sentences of all types:

Type 1: (Unless + present)

a. You'll be sick unless you stop eating. (= You will be sick if you don't stop eating) b. I won't pay unless you provide the goods immediately. (= If you don't provide them I won't pay) c. You'll never understand English unless you study this grammar carefully. (= You'll never understand if you don't study...)

Type 2: (Unless + past)

a. Unless he was very ill, he would be at work. b. I wouldn't eat that food unless I was really hungry. c. She would be here by now unless she was stuck in the traffic.

Type 3: (Unless + past perfect)

a. The elephant wouldn't have seen the mouse unless she'd had perfect eyesight. b. I wouldn't have phoned him unless you'd suggested it. c. They would have shot her unless she'd given them the money.