Form

Formula: If + past perfect, would have + past participle
If clause (unreal past condition)Main clause (imagined past result)
If I had studied harder,I would have passed.
If she hadn't been late,she would have got the job.
If it had rained,we wouldn't have gone out.
If they had left earlier,they would have caught the train.
Never use would in the if clause: ✓ "If I had known, I would have come." ✗ "If I would have known…"
'd = had or would? In the if clause, 'd = had: "If I'd known…" (= If I had known). In the main clause, 'd = would: "I'd have come" (= I would have come).

Use

The third conditional talks about imaginary situations in the past. The condition did NOT happen — we are imagining what the result would have been.

UseExample
Regret about the past If I had saved more money, I would have bought a house.
(I didn't save — I regret it)
Imagining a different outcome If she had taken the other job, she would have been happier.
Blaming / criticism If you hadn't been so careless, this wouldn't have happened.

Second vs Third Conditional

Second ConditionalThird Conditional
Hypothetical present/future Hypothetical past
If I had time, I would help.
(I don't have time now)
If I had had time, I would have helped.
(I didn't have time then)
If she lived here, she would walk to work. If she had lived here, she would have walked to work.

Exercises

Choose the correct third conditional form.

1. If I ___ the map, we wouldn't have got lost.

2. She would have got the job if she ___ better at the interview.

3. If they hadn't missed the flight, they ___ on time.

4. ___ he known about the problem, he would have fixed it immediately.

5. We would have come to the party if you ___ us.