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)

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

Type 2: (Unless + past)

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

Type 3: (Unless + past perfect)

  • Our marketing director would not have signed the contract unless she'd had the company legal expert present.
  • I wouldn't have phoned him unless you'd suggested it.
  • They would have shot her unless she'd given them the money.

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