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We make the third conditional by using the past perfect after 'if' and then 'would have' and the past participle in the second part of the sentence:

if + past perfect, ...would + have + past participle

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The third conditional is used to express the past consequence of an unrealistic action or situation in the past.

For example,

If he had studied harder, he would have passed the exam.

The first action (studying hard) did not happen. But in the case that he happened, the consequence was passing the exam. The third conditional is very similar to the second conditional. But while the second conditional refers to something unrealistic now or in the future, the third conditional refers to something unrealistic in the past.

We often use the third conditional to express regrets – describing things we are sorry happened or didn’t happen. For example,

If my alarm had gone off, I wouldn’t have been late to work.

If there hadn’t been so much traffic we wouldn’t have missed our flight.

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It talks about the past. It's used to describe a situation that didn't happen, and to imagine the result of this situation.

If she had studied, she would have passed the exam (but, really we know she didn't study and so she didn't pass)

If I hadn't eaten so much, I wouldn't have felt sick (but I did eat a lot, and so I did feel sick).

If we had taken a taxi, we wouldn't have missed the plane

She wouldn't have been tired if she had gone to bed earlier

She would have become a teacher if she had gone to university

He would have been on time for the interview if he had left the house at nine

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As with all conditionals, you can also invert this structure:

Would have + past participle if + past perfect.

I’d have come to the meeting if you’d told me about it. (I would have come to the meeting, if you had told me about it)

I wouldn’t have missed the meeting if you’d told me about it.

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The word would is often contracted to ‘d by native speakers. It’s also acceptable to use this in informal writing. And in speech it’s common to contract have to ‘ve in the third conditional. For example,

I’d've come to the meeting if you’d told me.

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However, we can’t write this, even in an informal context. It’s useful to be aware of it though, so you can identify it when native speakers say it.

As an alternative to would, we can complete the second part of a third conditional sentence with could. For example,

If I’d stayed at university, I could have got a masters degree.

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Here are some other examples of the third conditional:

He’d have got the job if he hadn’t been so nervous in the interview.

What would you have done if you’d been me?

If it hadn’t been snowing heavily, we’d have carried on skiing.

The company would have survived if there hadn’t been a recession.

Would you have accepted the offer if we’d reduced the price?

If you hadn’t invited me out, I’d have stayed in all day.

She wouldn’t have given you a fine if you’d apologized.

If they hadn’t won that match, the club would have fired the manager.

They could have stayed here if they hadn’t found any accommodation.

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Mixing the second and the third conditionals

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Mixing the second and the third conditionals

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It’s possible to combine the second and the third conditionals. There are two ways of doing that. We can either

Describe the present consequence of a past situation

(If + past simple, would have + past participle)
If I were adventurous, I’d have gone backpacking after university.

Describe the past consequence of a present situation

(If + past perfect, would + verb)
If we hadn’t missed the flight, we’d be in our hotel by now.

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If I’d studied for a year in the U.S, my English would be fluent now.

The roads wouldn’t be so icy if it hadn’t rained so much last night.

If she weren’t so shy, she’d have gone to the party on her own.

The fans would be miserable now if their team had been relegated.