We use the third conditional to talk about something in the past that could have been different.  We form the third conditional with if + past perfect in the condition and would + present perfect in the infinitive in the effect.


if + had + past participle, would + have + past participle


  1. If I had been home last night I would have helped you with your work.
  2. If people had known that DDT was dangerous, they wouldn’t have used


It is also common to use might and could instead of would to give the idea that the effect was a possible option.


  1. If they had seen him there, they might have invited him to go with them.
  2. If I had joined that music group in the summer of 1969, I might have become


There is a mixed conditional which combines the past with the present or future using if + past perfect with would + infinitive.  We consider them to be third conditionals since they involve changing an event in the past to create an unreal present.


  1. If you had learned this before, you wouldn’t be here now.
  2. If you hadn’t lost the tickets, we would be going to Hawai tomorrow.


As with the other conditionals, it is possible to reverse the order, use negatives or add additional conditions or effects.


  1. Jesse would have gone to dinner with his friend, if his girlfriend hadn’t invited him out.
  2. If I hadn’t gone on vacation to Hawai, I would never have met my wife.

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