The third conditional is used to describe an event or situation in the past that did not happen.
It can be used to talk about regrets or what could have happened if something had been different.

Positive: If + subject + had + past participle, subject + would/could/might + have + past participle
Negative: If + subject + had+ not + past participle, subject + would/could/might + have + past participle
Question: If + subject + had + past participle, would/could/might + subject + have + past participle?

Common uses:

1. Expressing regret or disappointment about something that did not happen in the past

• If I had studied more, I would have passed the exam.
• If I hadn’t been lazy, I would have learned how to play the guitar.
• If you had saved more money, would you have gone on holiday?

2. Speculating about the past and imagining different outcomes

• If we had left earlier, we would have caught the train.
• If we hadn’t missed the train, we wouldn’t have had to take the bus.
• If the train had been on time, would we have caught it?

3. Talking about hypothetical or imaginary situations in the past

• If I had won the lottery, I would have quit my job.
• If I hadn’t lost my job, I might have bought a house.
• If you had won the race,  would you have gotten a medal?