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.