Third Conditional
We use Third Conditional to talk about or write about past unreal situations, past situations that didn't actually happen. For example:
If I had studied harder, I would have passed the test.
So, I didn't study harder and I didn't pass the test, but this sentence describes a different past, an unreal past, an imagined past.
They would have bought it if they had had more money.
How we make Third Conditional
Third Conditionals have two parts, and the verb in each part is different. The “if” part uses a Past Perfect form, and the other part uses a “would have done” pattern. We separate the two parts with a comma.
If I had arrived earlier, I would have met him.