The Third Conditional talks about situations the past that did not happen, and their hypothetical results. Because the situation did not happen, there is no possible result.
We can use the Third Conditional for dreams, but with absolutely no chance of the dream coming true. For example: Yesterday you bought a lottery ticket but you didn’t win. But in your imagination you would say “If I had won the lottery, I would have bought an island.”
We can also use this condition for criticism or regret. For example: “If you had done your job properly, we wouldn’t be in this mess now.” or “If I had studied harder, I could have passed the exam.”
We use the Past Perfect to talk about the past condition. We use Would Have or Could Have or Might Have + Past Participle to talk about the imaginary result.
|If||you had called me,
||I could have come.|
|If||I hadn’t helped you,||you might have failed.|
|If||we had paid the rent,||we wouldn’t have been evicted.|
|If||she had told me you were coming,||I would have come too.|
|If||you had driven more carefully,||you wouldn’t have had an accident.|
|If||it had rained yesterday||what would you have done?|
|I could have told him||if||I had seen him.|
|They might have accepted your ideas||if||you had explained them better.|
|I would have finished hours ago||if||I hadn’t wasted so much time.|
|She would have gone||if||she had had time.|
|I would have done the same thing||if||I had been there.|
|What would you have said||if||he had asked you?|
Sometimes the “if” clause is implied. For example: I would have come. (but you didn’t ask me)
“I had” and “I would” are both contracted as “I’d”, so watch out for that.
In some varieties of English, the condition clause can use “would have” or “would’ve” but this is usually considered non-standard and is not usually used in more formal writing. For example: “If you would have told me, I would have done something” should probably read “If you had told me, I would have done something”.