Third Conditional Form : ( If + subject + past perfect, subject + would + have + past participle (verb 3) )

In English we use the third conditional to talk about imaginary situations in the past. They are often the opposite of what has actually happened. We talk about a condition in the past that did not happen. There is no possibility for this condition. The important thing about the third conditional is that both the condition and result are impossible now. 

Example Sentences :

• If he hadn't worn a bullet-proof vest, he would have been shot dead.(He did wear a vest and he wasn't shot dead)
• If I had studied for the exam, I would have passed. ( I didn't study and I didn't pass)
• If I hadn't gone to London, I wouldn't have met my boyfriend.
• If I had saved some money, I would have bought a house.

Sometimes, in English we use 'should have', 'could have', 'might have' instead of 'would have' to mean 'would perhaps'.

• If I had studied for the exam, I might have passed.
• If it had been me, I might have called the police.
• If you had bought a lottery ticket, you might have won.
• If I had met Michael Jackson, I might have asked him how to moonwalk! 

The 'if clause' can be first or second in the sentence.

Notes for use of third conditional :

In English we make questions in the third conditional with :

(Question word) + would + subject + have + past participle... + if + subject + past perfect simple?

• What would you have done if you'd seen him?

In English we can also use 'imagine' and 'suppose' instead of 'if' in third conditional questions.

• Imagine you'd seen him, what would you have done?

In English we can use a continuous form instead of a simple form to emphasize an action in progress.

• He wouldn't have seen her if he hadn't been jogging through the park. (variation of third conditional)