Third Conditional: Unreal Situations in the Past

Third Conditional: an unreal situation in the past (it would have been different, but it is too late).

Form: if + Past Perfect, would have + V3.

• If I had left earlier, I would have caught the train.
• If she had studied, she would have passed.
• We wouldn't have got lost if we had taken a map.

You can reverse the clauses: I would have called if I had known.

Informal speech sometimes has If I would have... — avoid it in standard English.