The third is used to talk about an imaginary past: something that did not happen, and what would have happened if it had. The form is if + past perfect, then would have + : If I had studied harder, I would have passed.
The form
The third has two clauses. In the if clause, use the past perfect: had plus the . In the main clause, use would have plus the past participle.
If + past perfect, would have + past participle. If the council had built more cycle lanes, fewer people would have used cars.
As with other conditionals, the clauses can change places. If the if clause comes first, use a comma. If it comes second, do not: Fewer people would have used cars if the council had built more cycle lanes.
- If I had left earlier, I would have caught the train. · I would have caught the train if I had left earlier.
- If they had checked the forecast, they would not have cancelled the trip. · They would not have cancelled the trip if they had checked the forecast.
What it means
The third is about the past, and about a past that did not happen. When you say If I had studied harder, I would have passed, you are saying that you did not study hard enough and you did not pass. You are imagining a different past.
Because the past cannot change, this conditional is often used to show regret, to criticise, or to explain how things could have been different. It is also useful in academic writing when you discuss past decisions and their effects.
- Regret: If I had saved more money, I would have bought a flat.
- Criticism: If the company had listened to its customers, it would not have lost so many sales.
- Explaining effects: If the dam had not been built, the valley would have flooded.
Modals other than would
You can use other in the main clause instead of would. Could have shows possibility: If we had left earlier, we could have avoided the traffic. Might have shows a weaker possibility: If she had taken the course, she might have found a better job.
The if clause stays the same. It always uses the past perfect. If you want the negative, put not after had: If they had not ignored the warning, the damage would have been smaller.
Third or second conditional?
The second is about an imagined present or future, and the third conditional is about an imagined past. If I had more time, I would learn the piano is a wish about now. If I had had more time last year, I would have learned the piano is about a past that did not happen.
Sometimes the two are mixed. If I had taken that job, I would be living in Dublin now uses a past condition and a present result. For most IELTS uses, the pure third conditional is enough, and you should learn it well before you try mixed forms.
Questions and negatives
For a question, put would before the subject in the main clause: Would more students have passed if the course had been shorter? Or put the if clause first: If the course had been shorter, would more students have passed?
For a negative, use hadn't or had not in the if clause and wouldn't have or would not have in the result. If the city had not raised fares, more people would have used the buses. Check that each negative matches your meaning, because not in the wrong clause reverses the idea.
Using it in IELTS
In Speaking Part 2, the third is a natural way to finish a story about a past experience. After you describe a trip, a course or a difficult decision, you can say If I had known how hard it would be, I would have prepared more. It shows that you can reflect on the past and gives the listener a reason to follow your story.
In Writing Task 2, you will usually use it for one purpose: to show what history might have looked like if a decision had been different. Keep it to one sentence, and check each part: had, the participle, would have, and the participle again. The structure has four parts, so errors are easy to make, and a quick check is worth the time.
Common mistakes
The most common error is putting would have in the if clause. We do not say If I would have known, I would have helped. Use If I had known.
The second is using the wrong participle. Learn the irregular verbs: If she had gone, if he had seen, if we had written. The third is leaving out have in the result: If I had asked, I would helped. Say would have helped.
In speech, had and would both shorten to 'd, so If I'd known, I'd have helped can sound confusing. Remember that the first 'd is had and the second is would. In writing, use the full forms until you are confident.
Who finds this hard?
- Chinese speakers: Chinese verbs do not change for tense, so the had plus participle pattern in the if clause has to be learned as a new form.
IELTS tip
In Speaking Part 3, the third lets you discuss how past decisions changed things, such as If governments had invested in public transport earlier, cities would have had less traffic. In Writing Task 2, use it sparingly and correctly, since a single accurate example shows range.
Frequently asked questions
What is the third conditional used for?
It is used for imaginary situations in the past, such as regrets, criticism and explaining how things could have been different.
Can I say If I would have known?
No. In standard written English, the if clause uses the past perfect: If I had known.
How is it different from the second conditional?
The second is about the present or future, and the third is about the past. Compare If I had money (now) and If I had had money (then).
Can I use could or might instead of would?
Yes. Could have and might have make the result less certain.
Sources
- Swan, M. (2016). Practical English Usage (4th ed.). Oxford University Press.
- Murphy, R. (2019). English Grammar in Use (5th ed.). Cambridge University Press.