The past is past and (sometimes unfortunately) it cannot be changed. All we can do is imagine how it might be different now if things had gone differently.

To do this, to describe unreal situations in the past, we use the Third Conditional.

If I had known the party was for your birthday, I would have got you a present.

But nobody told me this was a birthday party so I didn’t buy you a birthday present…

If you had sold your house back in November, you could have made a fortune.

Now the housing market has collapsed and your house is hardly worth anything…

If John had booked the table as I told him, now we would not be out in the rain looking for a restaurant.

But John did not reserve a table and now we are out in the pouring rain hoping to find another place to eat…

In other words, the third conditional is used to talk about regret or to criticize actions from the past which didn’t turn out well.

Form

The third conditional is formed by an if clause and a main clause. The order of these two clauses‏‎ can be inverted.

{if clause} + {main clause}

If I had left the house earlier, + I wouldn’t have missed the train.

{main clause} + {if clause}

I wouldn’t have missed the train + if I had left the house earlier.

Note that if the ‘if clause’ comes first, we usually use a comma.

Main Clause

The main clause in a third conditional sentence can contain any of the modal verbs‏‎: would, could or might according to the meaning we wish to convey followed by have and the past participle:

{modal verb} + have + {past participle}

I would have given you a lift if I had seen you.

We might have won if you had played better!

She could have been my wife if I hadn’t been an idiot.

If Clause

The if clause is made up of the subject followed by a verb in the past perfect.

if + {subject} + {past perfect}

If you had arrived earlier, you would have met him!

If they had found the gold, they would be rich now.

Note that sometimes the if clause can be implied rather than spoken.

I’d have done it for you.

I wouldn’t have said so.

Useful Links

Conditionals‏‎ in English Grammar – an overview of conditionals