Form

If + past perfect + would have + past participle

Functions and examples

We use the third conditional to imagine a different past.

If I had done my homework, my teacher wouldn't have shouted at me.
-- In reality, I didn't do my homework, and my teacher shouted at me. So we are imagining a different past

She would have passed her exam if she had studied more.
-- In reality she didn't study enough, and so she didn't pass her exam. So we are imagining a different past

Important points

1. We can use other modal verbs in place of 'would'.

If they'd come earlier, they could have got a seat.
-- In reality, they didn't come early, and they didn't get a seat. In our different past, there was a possibility of a seat

2. Mixed conditional: if + past perfect + would + bare infinitive
Sometimes we want to say that the result of an action is now.

If it hadn't rained, I would have gone to the beach.
-- In reality, it rained in the past, and I didn't go to the beach in the past

If it hadn't rained, I would be at the beach / would be sitting on the beach.
-- In reality, it rained in the past, and I am not at the beach now. This is a mixed conditional