The third conditional

We use the third conditional to talk about imagined past events: things that might have happened in the past, but didn’t happen.

If I’d known it was your birthday, I’d have bought you a present.
If the taxi had arrived on time, he wouldn’t have missed the plane.

A conditional sentence has two parts. In the third conditional, the if part is the imaginary situation in the past, and the main part is what could have happened (but didn’t happen) as a result. We make the third conditional with if + past perfect, and would have + past participle.

If I’d known it was your birthday… (This is the imaginary situation in the past)

I’d have bought you a present. (This is the imaginary result of the situation in the past)

The two parts can come in any order. When we write, we put a comma between the if part and the result part. You don’t use a comma when the result part comes first.

We use the past perfect in the if part to show the situation is imaginary and didn’t actually happen. The result part of the sentence tells us the imaginary result of this situation. 

If there had been any snow, we’d have gone skiing. (There wasn’t any snow; we didn’t go skiing.)

If it hadn’t been raining, we’d have had a picnic.(It was raining; we didn’t have a picnic.)

Form

Positive
If you’d asked me to marry you, I’d have said no.
We’d have been in trouble if we’d missed the last train.
Negative
She wouldn’t have become ill if she’d taken the medicine.
It would have been better if they hadn’t come to the party.
If you hadn’t been so friendly, I wouldn’t have talked to you.
Question
What would they have done if they’d lost their jobs?             
If I’d told him the truth, how would he have felt?
Short answerIn short answers, you use would/wouldn’t.
If you’d needed help, would you have asked me?
Yes, I would. / No, I wouldn’t.

Take note: past continuous

We can use the past continuous in the if part of the sentence.

If he’d been driving more carefully, he wouldn’t have had an accident. 
I wouldn’t have met my girlfriend if I’d been living abroad.

Take note: modals

We can use other modal verbs in the result part, for example might. Might shows we are less certain than when we use will.

We might have been happier if we’d bought the other house.If he hadn’t got up so late, he might not have missed the train.

Take note: ‘I wish …’and ‘If only …’

We use I wish or If only with the past perfect when we are sorry about something that happened in the past, and we imagine doing things differently.
 
I wish I’d stayed in bed this morning. (I’m having a bad day today.)

If only I’d stayed in bed this morning.

I wish I’d picked the other horse! (My horse didn’t win the race.)

If only I’d picked the other horse!

Spoken English

In the third conditional, we usually use a short form of had and had not when we speak: I had = I'd, I had not = I hadn’t. We also use a short form of would and would not: I would = I'd, he would = he'd, I would not = I wouldn’t, etc.

We’d have been unhappy if we’d lost the game.

We wouldn’t have been happy if we hadn’t won the game.
The third conditional is sometimes confusing because I’d can mean both I had and I would – so listen carefully! And remember that I’d in the if part is I had, and I’d in the result part is I would.

2) Double contractions

In spoken English, people often use contractions like this: I will becomes I'll and you would becomes you'd. Double contractions are when we shorten three words, like this:

I would have -> I'd've

could not have -> couldn't've

might not have -> mightn't've

must not have -> mustn't've

cannot have -> can't've

you would have -> you'd've

he would have -> he'd've

she would have -> she'd've

we would have -> we'd've

they would have -> they'd've