6.2.4 Third conditional
if + past perfect subjunctive | would + perfect infinitive
The third conditional describes an unreal situation with reference to the past. We imagine a hypothetical situation, something that did not happen:
If the driver had fastened his seat belt, he would probably have survived the accident.
Here If the driver had fastened his seat belt means that he did not fasten his seat belt.
In the if-clause, we use the past perfect subjunctive form of the verb, which is identical to the past perfect tense:
If we had booked our flight earlier, it would have been cheaper.
If you had taken a taxi, you would have caught the train.
If I had revised more, I would have done better on my exam.
Past perfect continuous subjunctive in the third conditional
The past perfect continuous subjunctive can be used in the if-clause of a sentence in the third conditional to express an unreal action in the past which is imagined as continuous:
If the driver had been wearing a seat belt, he would probably have survived the accident.
If I had been living in New York at the time, I would have gone to the show.
If I hadn't been working all day, I would have gone jogging.
Modals in the third conditional
Main clause
We can use modals other than would in the main clause of a sentence in the third conditional:
If I had had the right tools, I could have fixed my bike. (ability)
If the weather had been worse, they might not have reached the top. (possibility)
If-clause
We can use could + perfect infinitive in the if-clause to express ability or possibility:
If he could have got to the station on time, he might have caught the train. (ability)
If I could have known this would happen, I'd never have started dating him. (possibility)