Remember that we use the third conditional to talk about things that did not happen in the past. The ideas are for impossible past situations. The form is if + past perfect, would have + past participle.
- If I had seen Lindsay, I would have told her the news.
- If Karl had been free last week, I would have invited him for dinner.
- If it had rained on Sunday, we wouldn't have gone for our walk.
- Their teacher would have been disappointed if they hadn't passed their test.