The Third Conditional

(if + past perfect, ... would + have + past participle)
If I had gone to bed early, I would have caught the train.

If clause and main clause

We use the past perfect in the if-clause of a third conditional sentence to talk about an imaginary or hypothetical situation in the past, and we use would have, could have or might have + past participle in the main clause to talk about the result or consequence of that imaginary situation.

• If you had come to class more often, you would have passed the test.
• If he hadn’t taken his helmet, he could have died.
• If the jacket had been a bit cheaper, I might have bought it.

Comma

As with all conditional types, we use a comma after the if–clause when it goes at the beginning of the sentence, but we don’t use a comma when the if-clause goes at the end.

• If I hadn’t overslept, I wouldn’t have been late.
• I wouldn’t have been late if I hadn’t overslept.

Unless

We can also use unless in conditional sentences to mean if … (not)

• I wouldn’t have arrived on time unless I had taken a taxi.
• I wouldn’t have arrived on time if I hadn’t taken a taxi.