3 Third conditional

If + past perfect followed by would have is used when we want to imagine the consequences of events that happened, or didn't happen, or began to happen in the past:

If I had known about the delay, I wouldn't have got to the airport so early.

If there hadn't been a mix-up with our booking, we'd have had a room with a view.

If you had reminded me to confirm the booking, I'd have written a letter.

Remember that 'd is the short form of both had and would:

If he'd (he had) reminded me, I'd have (I would have) arrived on time.

If I'd had (I had had) more time, I'd have (would have) finished my work.

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