Introduction

We use the third conditional to describe unreal situations in the past and the imagined results. Both the situation and the result exist in the past; the situation did not happen so the result is impossible:

    If I had woken up earlier, I wouldn’t have missed the train.     

• "If I had woken up earlier" is the unreal situation.
• "I wouldn’t have missed the train" is the imagined result.

    The real situation and result: I didn’t wake up earlier so I missed the train.

Examples:

• If she hadn't got the job, she would have been very disappointed.
  The real situation and result: she got the job so she wasn’t disappointed.
   
• If they had known about the problems with the trains, they would have driven.
  The real situation and result: they didn’t know about the problems with the trains so they didn’t drive.
   
• Would I have got sick if I hadn’t eaten the fish?
  The real situation and result: I ate the fish. Was that why I got sick?

Form

We form the third conditional with a conditional clause and a main clause:

• Conditional clause: "if" + past perfect
• Main clause: "would have" + past participle

The order of the clauses does not matter: the meaning is the same.

If we put the conditional clause before the main clause, we use a comma:

Conditional clause	Main clause
"If" + past perfect + comma	"would have" + past participle
If I’d woken up earlier,	I wouldn’t have missed the train.
If she hadn’t got the job,	she would have been very disappointed.
If they had known about the problems with the trains,	they would have driven instead.
If I hadn’t eaten the fish,	would I have got sick?

We do not use a comma if we put the main clause before the conditional clause:

Main clause	Conditional clause
"would have" + past participle	"if" + past perfect
I wouldn’t have missed the train	if I had woken up earlier.
She would have been very disappointed	if she hadn’t got the job.
They would have driven	if they had known about the problems with the trains.
Would I have got sick	if I hadn’t eaten the fish?

• We can contract the subject pronoun and "would": "I’d" / "you’d" / "we’d" / "they’d" / "she’d" / "he’d" / "it’d"
   
• We can contract "would" and "have": would’ve
   
• We can use the past perfect continuous in the conditional clause:
  If I’d been looking more carefully, I would have noticed the mistake.
  I didn’t notice the mistake because I wasn’t looking carefully enough.

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Practice

• They’d have passed their exams if they harder. (study)
   
• If he had followed the directions, he on time. (arrive)
   
• Would you have guessed the ending of the film if you the book. (not read)
   
• Sarah would to Paris if she’d had enough time. (go)
   

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Further practice

Complete the sentences to form the third conditional by choosing the correct answer from a, b, c or d:

• If he had known it was her birthday, he would ____ her a present.


have buy had bought have bought had buy



• They wouldn’t have gone if they ____ it would be so crowded.


'd knew had known 'd know had knows



• Would you ____ to the conference if the company hadn’t paid for the tickets?


have gone had gone have went had went



• I would ____ meat if I had known that you are vegetarian.


not had cooked n't have cook not had cook n't have cooked



• If I ____ the menu, I wouldn’t have gone to the restaurant.


had saw have saw had seen have seen 



• If they ____ a map, would they have got lost?


have bought had bought had buy have buy

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Use of "could", "might"

We can use "could" or "might" instead of "would" in the main clause of the third conditional.

• If they had known about the problems with the trains, they would have driven.
  The main clause with "would" describes a certain result.
   
• If they had known about the problems with the trains, they could have driven.
  The main clause with "could" describes a possible result as an ability to do something.
   
• If they had known about the problems with the trains, they might have driven.
  The main clause with "might" describes a possible result.